patch 8.0.1708: mkdir with 'p' flag fails on existing directory

Problem:    Mkdir with 'p' flag fails on existing directory, which is
            different from the mkdir shell command.
Solution:   Don't fail if the directory already exists. (James McCoy,
            closes #2775)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index b0170b8..1868f4e 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -6138,6 +6138,8 @@
 		Example: >
 			:call mkdir($HOME . "/tmp/foo/bar", "p", 0700)
 <		This function is not available in the |sandbox|.
+		There is no error if the directory already exists and the "p"
+		flag is passed (since patch 8.0.1708).
 		Not available on all systems.  To check use: >
 			:if exists("*mkdir")
 <