(DOCUMENT-22) Clarify matching of crontab entries

This updates the documentation to be consistent with behavior introduced in
commit da9162c.
(maint) Typo fixes: repeated words (e.g. "the the")
(maint) Revise help for puppet config subcommand

Mostly adding context about the idea of sections, inc. list of available sections.
(PUP-1275) Never try to fork on Windows

In 3.0.2, in commit 4d1a2f83d9 for #17361 restored the ability for *nix
agents to apply a catalog in a forked child process. But the change
caused Windows agents to also try to fork if run without the `--onetime`
flag.

This regression means you can't run Windows agents with `--listen` and
kick them.

This commit adds a `can_fork?` method to ensure we only attempt to fork
if we both can and should fork. There is no change in behavior for *nix
agents.
Merge pull request #2304 from Sharpie/DOCUMENT-22-cron

(DOCUMENT-22) Clarify matching of crontab entries
Merge pull request #2362 from nfagerlund/docs_config_command

(maint) Revise help for puppet config (plus another minor doc commit)
Merge pull request #2398 from joshcooper/issue/master/PUP-1275-windows-kick

(PUP-1275) Never try to fork on Windows
Merge branch 'master' of https://github.com/puppetlabs/puppet