Add all keywords to keyword docs

This commit gives each stable keyword a short entry in the "Keywords"
section in the docs for `std`. The newly added entries are a single
summary line and a note that the documentation is not yet complete.  I
changed some of the existing summary lines for consistency's sake. Each
line is either a verb phrase ("name the type of a trait object" for
`dyn`), or an object ("A value of type `bool` representing logical true"
for `true`). I tried to avoid using the keyword itself or the word
"keyword" in the summary.

Later PRs can flesh out each keyword with an example of each
context in which a keyword can appear and a link to the rust book.

Keywords which are not close to stable rust such as `box` (which is
getting unstabilized) or `try` are ignored in this PR.

Author: ecstatic-morse