.mailmap: Remove stale SVN references

Also explain the true purpose of this file.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

.mailmap

@@ -1,8 +1,19 @@
-# This file exists to help map usernames to proper names and email addresses
-# in the Open MPI github mirror of the canonical SVN repository.  The github
-# mirror can be found here:
+# This file exists to help consolidate names and email addresses
+# (e.g., when people accidentally commit with an incorrect or local
+# email address).  Two common use cases:
 #
-#   https://github.com/open-mpi/ompi-svn-mirror
+# 1. Consolidate multiple email addresses from a single person.
+#    Example: one commit from John Smith is from
+#    <john.smith@his.work.com> and another is from
+#    <jsmith@workstation.his.work.com>, and a third is from
+#    <rocketman9982@users.noreply.github.com>.  But they're all from
+#    the same John Smith person.
+#
+# 2. Consolidate misspellings / altername names from a single person.
+#    Example: one commit is from "John Smith" and another is from
+#    "John Smith, CONTRACTOR", and third is from "RocketMan 9982".  But
+#    these are all really the same person, who can be listed once in
+#    AUTHORS as "John Smith".
 #
 # The format of this file is documented in git-shortlog(1).  Specifically,
 # a line like this:
@@ -12,12 +23,11 @@
 # means that when git sees "commit@email.xx" it will display
 # "Proper Name <proper@email.xx>" instead in certain circumstances.  Those
 # circumstances include:
+#
 #   - git shortlog
 #   - git blame
 #   - git log --format=tformat:"%aN <%aE>"   (and similar)
 #
-# A copy of this file should be present on each branch in SVN which is being
-# tracked in the Git mirror.
 
 # Jeff accidentally stomped on his $HOME/.gitconfig for a short while:
 Jeff Squyres <jsquyres@cisco.com> --quiet <--quiet>