Change keywords field to be comma separated

With a note about the change.

Discussion on distutils-sig:
https://mail.python.org/archives/list/distutils-sig@python.org/thread/LFTLGMIIB5PRG7NUUIGGGX4COGX53DMR/

Author: takluyver

source/specifications/core-metadata.rst

@@ -268,12 +268,19 @@ Keywords
 
 .. versionadded:: 1.0
 
-A list of additional keywords to be used to assist searching
-for the distribution in a larger catalog.
+A list of additional keywords, separated by commas, to be used to assist
+searching for the distribution in a larger catalog.
 
 Example::
 
-    Keywords: dog puppy voting election
+    Keywords: dog,puppy,voting,election
+
+.. note::
+
+   The specification previously showed keywords separated by spaces,
+   but distutils and setuptools implemented it with commas.
+   These tools have been very widely used for many years, so it was
+   easier to update the specification to match the de facto standard.
 
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