Clarified when content_type is optional

Added a comment clarifying that `long_description_content_type` in `setup()`
is only optional if the `long_description` is written in RST and a quote
from relevant documentation.

setup.py

@@ -60,6 +60,11 @@
     # Denotes that our long_description is in Markdown; valid values are
     # text/plain, text/x-rst, and text/markdown
     #
+    # Optional if long_description is written in rst but required for
+    # plain-text or Markdown; if unspecified, "applications should attempt to
+    # render [the long_description] as text/x-rst; charset=UTF-8 and fall back
+    # to text/plain if it is not valid rst" (see link below)
+    #
     # This field corresponds to the "Description-Content-Type" metadata field:
     # https://packaging.python.org/specifications/core-metadata/#description-content-type-optional
     long_description_content_type='text/markdown',  # Optional