RelNotes: mention "log: make --regexp-ignore-case work with --perl-regexp"

To inform users that they can use --regexp-ignore-case now, and that
existing scripts which relied on that + PCRE may be buggy. See
9e3cbc59d5 ("log: make --regexp-ignore-case work with --perl-regexp",
2017-05-20).

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

Documentation/RelNotes/2.14.0.txt

@@ -120,6 +120,9 @@ UI, Workflows & Features
  * "git log" learned -P as a synonym for --perl-regexp, "git grep"
    already had such a synonym.
 
+ * "git log" didn't understand --regexp-ignore-case when combined with
+   --perl-regexp. This has been fixed.
+
 Performance, Internal Implementation, Development Support etc.
 
  * The default packed-git limit value has been raised on larger