Remove some duplication in "Provider information"

Environment marker usage and optionality is described in "Providers"
section. Let's limit "Provider information" only to technical field
details themselves.

Signed-off-by: Michał Górny <mgorny@quansight.com>

Author: mgorny

peps/pep-0817.rst

@@ -1455,21 +1455,15 @@ A provider information dictionary MAY contain the following keys:
 
 - ``enable-if: str``: An :ref:`environment marker
   <dependency-specifiers-environment-markers>` defining when the plugin
-  should be used. If the environment marker does not match the running
-  environment, the provider will be disabled and the variants using its
-  properties will be deemed incompatible. If not provided, the plugin
-  will be used in all environments.
+  should be used.
 
 - ``install-time: bool``: Whether this is an install-time provider.
   Defaults to ``true``. ``false`` means that it is an AoT provider
   instead.
 
 - ``optional: bool``: Whether the provider is optional. Defaults
   to ``false``. If it is ``true``, the provider is
-  considered optional and SHOULD NOT be used unless the user opts in to
-  it, effectively rendering the variants using its properties
-  incompatible. If it is ``false``, the provider is considered
-  obligatory.
+  considered optional.
 
 - ``plugin-api: str``: The API endpoint for the plugin. If it is
   specified, it MUST be an object reference as explained in the `API