Add documentation for platform options and expose to Godoc

[djdv]

KeyValue's documentation says "See platform docs for more details.", but I didn't see any in Godoc.

When looking through the source I found a helpful table, but it was not being exposed to the documentation generator.
Some of the platform specific options were missing as well.

I moved the table so that godoc associates it with KeyValue, and filled in the platform blanks I found.
(I think it's all of them, but I'm not positive)

In addition, I noticed UserService is checked on more platforms than just OS X. So I moved it to the POSIX section.
I considered moving some of the POSIX options out to the Linux section since they're only checked there, but figured platform support may be added for them later. So I kept them as is.

There's also some minor linter complaints addressed in the comments like adding periods, and a small spelling correction.
These can be split out or removed from the patch if desired.

This is how things are rendering for my local instance:

compared to the current release:
https://pkg.go.dev/github.com/kardianos/service@v1.2.0#Config
+
https://pkg.go.dev/github.com/kardianos/service@v1.2.0#KeyValue

[coveralls]

Coverage remained the same at 12.799% when pulling 58ccf27 on djdv:add/docs into ef35c56 on kardianos:master.

[djdv]

(Force pushed over a typo I missed "reaload" -> "reload")