RTD config: document and fix

.readthedocs.yml

@@ -7,7 +7,7 @@ version: 2
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
-  configuration: doc/conf.py
+  configuration: doc/source/conf.py
 
 # Optionally build your docs in additional formats such as PDF and ePub
 formats: all
@@ -16,4 +16,10 @@ formats: all
 python:
   version: 3.7
   install:
+    # WARNING: This requirements file will be installed without the pip
+    #          --upgrade flag in an existing environment. This means that if a
+    #          package is specified without a lower boundary, we may end up
+    #          accepting the existing version.
+    #
+    #          ref: https://github.com/readthedocs/readthedocs.org/blob/0e3df509e7810e46603be47d268273c596e68455/readthedocs/doc_builder/python_environments.py#L335-L344
     - requirements: doc/doc-requirements.txt