Add OptionalOrNullptr to optional.md docs

This patch adds a mention of OptionalOrNullptr to the optional.md docs.

Change-Id: I7d09afa336bcd06a11756b5d655370d162b80318
Reviewed-on: https://chromium-review.googlesource.com/965585Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543725}

docs/optional.md

@@ -109,7 +109,9 @@ undefined value when the expected value can't be negative.
 It is recommended to not use `base::Optional<T>` as a function parameter as it
 will force the callers to use `base::Optional<T>`. Instead, it is recommended to
 keep using `T*` for arguments that can be omitted, with `nullptr` representing
-no value.
+no value. A helper, `base::OptionalOrNullptr`, is available in
+[stl_util.h](https://code.google.com/p/chromium/codesearch#chromium/src/base/stl_util.h)
+and can make it easier to convert `base::Optional<T>` to `T*`.
 
 Furthermore, depending on `T`, MSVC might fail to compile code using
 `base::Optional<T>` as a parameter because of memory alignment issues.