Made apparent the functionality of the ftoptions plugin (#2321)

Made apparent the functionality of the ftoptions plugin (#2321)

* Clarified meaning of indentchar setting

The description "sets the indentation character" combined with the default value of a space led me to believe that this was a way to set a preference for tabs/spaces and choose a number of spaces per indentation all at once. I've updated the description to try to make its true function clearer.

* Added note on rmtrailingws

This behavior was unexpected for me, so it's probably good to let other users know which option has precedence.

* Added details to help command

Initially I kept trying to use `help <command-name>` rather than `help commands`

* Added warning about ftoptions and tabstospaces

The current description for ftoptions states that it "alters some default options depending on the filetype", which hints at this behavior, but does not explicitly state it.

* Clarified specific functionality of ftoptions