MM-31575 ENV var license override (#4204)

* MM-31575 ENV var license override

Documentation for: https://mattermost.atlassian.net/browse/MM-31575

Updated:
- Self-Managed Admin Guide > Configure Mattermost > Configuration Settings
   - Changed the following bolded lines to proper headings: "Configuration in Database", "Environment Variables", and "Load Custom Configuration Defaults"
   - Added new subsection "Override Mattermost License File"

* Fixed typo

"In a high-availability environment, using an environment variable to override a server license only affect[s] the individual app server and doesn't propagate to other servers in the cluster."

* Update source/administration/config-settings.rst

Co-authored-by: Justine Geffen <justinegeffen@users.noreply.github.com>

* Update source/administration/config-settings.rst

Co-authored-by: Justine Geffen <justinegeffen@users.noreply.github.com>

Co-authored-by: Justine Geffen <justinegeffen@users.noreply.github.com>

Author: cwarnermm

source/administration/config-settings.rst

@@ -8,15 +8,17 @@ Mattermost configuration settings are maintained in the ``config.json`` configur
 
 Mattermost must have write permissions to ``config.json``, otherwise changes made in the System Console will have no effect.
 
-On new installations starting from version 5.14, the ``default.json`` file used to create the initial ``config.json`` has been removed from the binary and replaced with a build step that generates a fresh ``config.json``. This is to ensure the initial configuration file has all the correct defaults provided in the server code. Existing ``config.json`` files are not affected by this change.
+On new installations starting from v5.14, the ``default.json`` file used to create the initial ``config.json`` has been removed from the binary and replaced with a build step that generates a fresh ``config.json``. This is to ensure the initial configuration file has all the correct defaults provided in the server code. Existing ``config.json`` files are not affected by this change.
 
-**Configuration in Database**
+Configuration in Database
+--------------------------
 
 Storing configuration in the database is supported in v5.10 and later.  Please see more information on how to set this up `here <https://docs.mattermost.com/administration/config-in-database.html>`_.
 
-**Environment Variables**
+Environment Variables
+---------------------
 
-Starting from Mattermost version 3.8, you can use environment variables to manage the configuration. Environment variables override settings in ``config.json``. If a change to a setting in ``config.json`` requires a restart for it to take effect, then changes to the corresponding environment variable also require a server restart.
+Starting from Mattermost v3.8, you can use environment variables to manage the configuration. Environment variables override settings in ``config.json``. If a change to a setting in ``config.json`` requires a restart for it to take effect, then changes to the corresponding environment variable also require a server restart.
 
 The name of the environment variable for any setting can be derived from the name of that setting in ``config.json``. For example, to derive the name of the Site URL setting:
 
@@ -37,14 +39,26 @@ For any setting that is not set in ``config.json`` or in environment variables,
 .. warning::
    Database connection strings for the database read and search replicas need to be formatted using `URL encoding <https://www.w3schools.com/tags/ref_urlencode.asp>`__. Incorrectly formatted strings may cause some characters to terminate the string early, resulting in issues when the connection string is parsed.
 
-**Load Custom Configuration Defaults**
+Override Mattermost License File
+--------------------------------
+
+Starting from Mattermost v5.26, you can use an environment variable to override any license in the database or file configuration without replacing those licenses.
+
+When starting the server, specify the license key as ``MM_LICENSE`` with the contents of a license file.
+
+.. note::
+   If ``MM_LICENSE`` is set to a non-empty string, but the license specified is not valid, the Mattermost server will be started without a license.
+   
+   In a High Availability deployment, using an environment variable to override a server license only affects the individual app server and doesn't propagate to other servers in the cluster.
+
+Load Custom Configuration Defaults
+----------------------------------
 
 Starting from Mattermost v5.30, you can load a set of custom configuration defaults using an environment variable. This custom configuration applies only if the values are not already present in the current server configuration.
 
 1. Create a JSON file that contains the custom configuration defaults. For example, ``custom.json``.
 2. When starting the server, point the custom defaults environment variable to the defaults file: ``MM_CUSTOM_DEFAULTS_PATH=custom.json``.
 
-
 .. contents::
   :depth: 2
   :local: