use namespace base for base::make_optional in docs

docs/optional.md

 # base::Optional
 
 `base::Optional<T>` is a container that might contain an instance of `T`.
 
 [TOC]
 
 ## History
 
 [base::Optional<T>](https://code.google.com/p/chromium/codesearch#chromium/src/base/optional.h)
 is an implementation of [std::optional<T>](http://en.cppreference.com/w/cpp/utility/optional),
@@ skipping 45 matching lines @@
 base::Optional<int> opt_2 = 2;
 
 opt_1 == opt_2; // false
 opt_1 = 1;
 
 opt_1 <= opt_2; // true
 opt_1 == 1; // true
 opt_1 == base::nullopt_t; // false
 ```
 
-`base::Optional<T>` has a helper function `make_optional<T&&>`:
+`base::Optional<T>` has a helper function `base::make_optional<T&&>`:
 
 ```C++
-base::Optional<int> opt = make_optional<int>(GetMagicNumber());
+base::Optional<int> opt = base::make_optional<int>(GetMagicNumber());
 ```
 
 Finally, `base::Optional<T>` is integrated with `std::hash`, using
 `std::hash<T>` if it is not empty, a default value otherwise. `.emplace()` and
 `.swap()` can be used as members functions and `std::swap()` will work with two
 `base::Optional<T>` objects.
 
 ## How is it implemented?
 
 `base::Optional<T>` is implemented with a union with a `T` member. The object
@@ skipping 19 matching lines @@
 
 ## When not to use?
 
 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.
 
 Furthermore, depending on `T`, MSVC might fail to compile code using
 `base::Optional<T>` as a parameter because of memory alignment issues.