regexp/syntax: clarify that \Z means Perl's \Z

Fixes #14793

Change-Id: I408056d096cd6a999fa5e349704b5ea8e26d2e4e
Reviewed-on: https://go-review.googlesource.com/23201Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/regexp/syntax/doc.go

@@ -66,7 +66,7 @@ Grouping:
 
 Empty strings:
   ^              at beginning of text or line (flag m=true)
-  $              at end of text (like \z not \Z) or line (flag m=true)
+  $              at end of text (like \z not Perl's \Z) or line (flag m=true)
   \A             at beginning of text
   \b             at ASCII word boundary (\w on one side and \W, \A, or \z on the other)
   \B             not at ASCII word boundary