std: Remove String::from_owned_str as it's redundant

[breaking-change]
std: Remove String's to_owned
std: Rename strbuf operations to string

[breaking-change]
doc: Fix link to string

This was missed in 553074506ecd139eb961fb91eb33ad9fd0183acb
auto merge of #14414 : richo/rust/features/nerf_unused_string_fns, r=alexcrichton

This should block on #14323