std: uniform modules titles for doc

This commit uniforms the short title of modules provided by libstd,
in order to make their roles more explicit when glancing at the index.

Signed-off-by: Luca Bruno <lucab@debian.org>
auto merge of #11134 : lucab/rust/lucab/libstd-doc, r=cmr

Uniform the short title of modules provided by libstd, in order to make their roles more explicit when glancing at the index.