general: Reformat documentation with better formatting and wording

General purpose reformating and rewording in (a few places). This should
also make markdown linting happier.

Prefer back-ticks wherever useful and use `console` for code blocks to
be executed on terminal.

Also, update references links wherever possible (including update from
'http://' to 'https://').

modules/gpg/README.md

@@ -1,13 +1,14 @@
-GPG
-===
+# GPG
 
 Provides for an easier use of [GPG][1] by setting up [gpg-agent][2].
 
+## Settings
+
 ### SSH
 
 To enable OpenSSH Agent protocol emulation, and make `gpg-agent` a drop-in
 replacement for `ssh-agent`, add the following line to
-*~/.gnupg/gpg-agent.conf*:
+*`~/.gnupg/gpg-agent.conf`*:
 
 ```conf
 enable-ssh-support
@@ -16,13 +17,12 @@ enable-ssh-support
 When OpenSSH Agent protocol emulation is enabled, this module will load the SSH
 module for additional processing.
 
-Authors
--------
+## Authors
 
 *The authors of this module should be contacted via the [issue tracker][3].*
 
-  - [Sorin Ionescu](https://github.com/sorin-ionescu)
+- [Sorin Ionescu](https://github.com/sorin-ionescu)
 
-[1]: http://www.gnupg.org
-[2]: http://linux.die.net/man/1/gpg-agent
+[1]: https://www.gnupg.org
+[2]: https://linux.die.net/man/1/gpg-agent
 [3]: https://github.com/sorin-ionescu/prezto/issues