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

@@ -53,6 +53,11 @@
     # Often, this is the same as your README, so you can just read it in from
     # that file directly (as we have already done above)
     #
+    # 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" metadata field:
     # https://packaging.python.org/specifications/core-metadata/#description-optional
     long_description=long_description,  # Optional