Improvements to :help

[bmatsuo]

The :help command needs to be improved in several important ways

• [x] Support for non-command help topics (e.g. "syntax"). These need to be in a separate list as not to be confused with commands. (resolution: dc07463424c52fd2ba0d96488e7a1921c7ddc06a)
• [x] Fixed (sorted) order for list entries. (resolution: dc07463424c52fd2ba0d96488e7a1921c7ddc06a)
• [x] Tag line description for each command (e.g. "peek -- apply filter without pushing it on the stack"). (resolution: 71a542d2dcf693528173e7b4c76efa96f1665f99)

[bmatsuo]

This is basically done. Command help could probably be improved by running it all through "go/doc", as is done with help topics. That can happen later though.