user-manual: update git-gc discussion

It appears git-gc will no longer prune automatically, so we don't
need to tell people not to do other stuff while running it.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>

Documentation/user-manual.txt

@@ -1447,16 +1447,14 @@ On large repositories, git depends on compression to keep the history
 information from taking up to much space on disk or in memory.
 
 This compression is not performed automatically.  Therefore you
-should occasionally run
+should occasionally run gitlink:git-gc[1]:
 
 -------------------------------------------------
 $ git gc
 -------------------------------------------------
 
-to recompress the archive and to prune any commits which are no
-longer referred to anywhere.  This can be very time-consuming, and
-you should not modify the repository while it is working, so you
-should run it while you are not working.
+to recompress the archive.  This can be very time-consuming, so
+you may prefer to run git-gc when you are not doing other work.
 
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