Keyword doc of `self` and `Self` points to the same file on windows.

[crlf0710]

Since windows filesystem is case insensitive, when installing rust-docs on windows, the self and Self pages conflicts, leaving only one of them accessible.

This is a specific case of #25879, but since it's in rust standard library. I think it deserves its own issue (and priority).

[GuillaumeGomez]

There was a first attempt to fix this issue here, but not sure if this is the right approach.

[rbtcollins]

For completeness - they point to the same file on MacOS X as well.