(maint) Fix various typos and problems in docs

Some typos, as per usual from my drafts. This also removes some
references to the injection feature, which is only part of internal
functions. Since that API is going to probably be going through some
more changs, I decided to simply remove the information rather than have
it keep becoming out of date.
(maint) Remove reference to README_DEVELOPER

The README_DEVELOPER.md document went away a while ago. Yard was issuing
a warning because it was told about that file specifically, but it
didn't exist.
(maint) Get Puppet::Pops::Functions to show in yard

Since Pops and friends were not marked with an @api tag they were not
showing up in the yard output. This changes them so that they are marked
@api public, with the caveat that it is still experimental.