Add a tidy check for acceptable rustdoc header

[pickfire]

I saw quite a few headers and I think it would be nice for tidy to keep track of them for consistency, maybe even do suggestion. For example, # Example and # Examples, we may accidentally use # Example sometimes which is not consistent with the rest of the documentation.

I also saw that sometimes there are # Safety: rather than # Safety. Maybe we could suggest them based on edit distance on the possible choices for headers we specified in tidy?

CC @rust-lang/docs

[jyn514]

@pickfire just to clarify: are you suggesting this as a general rustdoc lint? Or only for the standard library itself?

[pickfire]

Standard library itself, not as a rustdoc lint but a tidy lint. @GuillaumeGomez made me realized this, sometimes I almost made the same mistake but I took some time to check out how other parts did it.