RelNotes: mention "log: add -P as a synonym for --perl-regexp"

To inform users that they can use the short form now. See
7531a2dd87 ("log: add -P as a synonym for --perl-regexp", 2017-05-25).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/RelNotes/2.14.0.txt

@@ -117,6 +117,8 @@ UI, Workflows & Features
  * "git pull --rebase --recurse-submodules" learns to rebase the
    branch in the submodules to an updated base.
 
+ * "git log" learned -P as a synonym for --perl-regexp, "git grep"
+   already had such a synonym.
 
 Performance, Internal Implementation, Development Support etc.