docs: sort configuration variable groupings alphabetically

docs: sort configuration variable groupings alphabetically

By and large, variable groupings in Documentation/config.txt are sorted
alphabetically, though a few are not. Those outliers make it more
difficult to find a specific grouping when quickly running an eye over
the list to locate a variable of interest. Address this shortcoming by
sorting the groupings alphabetically.

NOTE: This change only sorts the top-level groupings (i.e. "core.*"
comes after "completion.*"); it does not touch the ordering of variables
within each group since variables within individual groups might
intentionally be ordered in some other fashion (such as
most-common-first or most-important-first).

Reported-by: Bruno Haible <bruno@clisp.org>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>