Update to "disallowed minor code changes"

Added section about minor typos in comments being disallowed,
following the discussion at the Glance meeting on 3 November.

Change-Id: Ic6c6369895a7095c01c1810358f4355c5cc8f0fa

doc/source/contributing/minor-code-changes.rst

@@ -18,6 +18,36 @@ scripts confuses operators and administrators -- we only want them to notice
 serious problems.  Their preference must take precedence over fixing spell
 errors.
 
+Typographical errors in comments
+--------------------------------
+
+Comments are not user-facing.  Correcting minor misspellings or grammatical
+errors only muddies the history of that part of the code, making ``git blame``
+arguably less useful.  So such changes are likely to be rejected.  (This
+prohibition, of course, does not apply to corrections of misleading or unclear
+comments, or for example, an incorrect reference to a standards document.)
+
+Misspellings in code
+--------------------
+
+Misspellings in function names are unlikely to be corrected for the "historical
+clarity" reasons outlined above for comments.  Plus, if a function is named
+``mispelled()`` and a later developer tries to call ``misspelled()``, the
+latter will result in a NameError when it's called, so the later developer will
+know to use the incorrectly spelled function name.
+
+Misspellings in variable names are more problematic, because if you have a
+variable named ``mispelled`` and a later developer puts up a patch where an
+updated value is assigned to ``misspelled``, Python won't complain.  The "real"
+variable won't be updated, and the patch won't have its intended effect.
+Whether such a change is allowed will depend upon the age of the code, how
+widely used the variable is, whether it's spelled correctly in other functions,
+what the current test coverage is like, and so on.  We tend to be very
+conservative about making changes that could cause regressions.  So whether a
+patch that corrects the spelling of a variable name is accepted is a judgment
+(or is that "judgement"?) call by reviewers.  In proposing your patch, however,
+be aware that your reviewers will have these concerns in mind.
+
 Tests
 -----