git-gc.txt: expand discussion of races with other processes

In general, "git gc" may delete objects that another concurrent process
is using but hasn't created a reference to.  Git has some mitigations,
but they fall short of a complete solution.  Document this in the
git-gc(1) man page and add a reference from the documentation of the
gc.pruneExpire config variable.

Based on a write-up by Jeff King:

http://marc.info/?l=git&m=147922960131779&w=2

Signed-off-by: Matt McCutchen <matt@mattmccutchen.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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