Add documentation to more `From::from` implementations.

For users looking at documentation through IDE popups, this gives them
relevant information rather than the generic trait documentation wording
“Performs the conversion”. For users reading the documentation for a
specific type for any reason, this informs them when the conversion may
allocate or copy significant memory versus when it is always a move or
cheap copy.

Notes on specific cases:
* The new documentation for `From<T> for T` explains that it is not a
  conversion at all.
* Also documented `impl<T, U> Into<U> for T where U: From<T>`, the other
  central blanket implementation of conversion.
* I did not add documentation to conversions of a specific error type to
  a more general error type.
* I did not add documentation to unstable code.

This change was prepared by searching for the text "From<... for" and so
may have missed some cases that for whatever reason did not match. I
also looked for `Into` impls but did not find any worth documenting by
the above criteria.

0 files changed, 0 insertions, 0 deletions