Documentation for deprecated methods

[jakebolewski]

We need a standardized way to document that a method is deprecated. v0.4 had a lot of deprecations but the only warning was emitted at runtime when the function was called. None of the docs were updated and no warning was thrown for these methods when calling help?>. When deprecated functions are removed the docs could persist for 1 more release cycle.

What we need going forward:

• A standardized way of formatting that a method is deprecated in its docstring.
• An expectation that when a method is deprecated, a specific docstring is updated.
• A way for the doc system to introspect that a particular method / type is deprecated.

[JeffBezanson]

Related, needs similar reflection features: #3526

[ScottPJones]

Should things that were deprecated in v0.4 start to be removed totally in master (v0.5-dev?).
I noticed the documentation problem yesterday:

help?> isvalid
search: isvalid is_valid_utf8 is_valid_char is_valid_utf32 is_valid_utf16

  isvalid(T, value) -> Bool
...
help?> is_valid_char
search: is_valid_char is_valid_ascii is_valid_utf8 is_valid_utf32 is_valid_utf16