Document how to stabilize a library feature (#1036)

* Move 'force-unstable-if-unmarked' to the bootstrapping chapter

* Document how to stabilize a library feature

Note that features can't be stabilized until they go through FCP and
that FCP happens on the tracking issue, not the PR.

* Fix wrong glob

By default `**` behaves the same as two `*` side by side, i.e. it only
globs file paths, not directories. `shopt -s globstar` needs to be set
for it to mean a directory. I didn't notice this before now because
`globstar` is set by default in interactive mode, but not otherwise.