doc: rewrite the style section of the manpage

[ThomasAdam]

The current section on Controlling Window Styles in fvwm3all, and hence
the generated fvwm3styles manpage is rather difficult to read.

The style options are currently separated into groups via / which were meant
to provide some sort of grouping, but over time, this was never going to be
sustainable.

To address this, it would be good to group the style options together, such
that they relate to:

• Focus Policies
• Window movement:
  ** Raising
  ** Resizing
  ** Maximising
  ** Shading
• Window placement
• Window decorations:
  ** Borders
  ** Handles
  ** CornerWidth
  ** etc...

Note that things like tables could be used to help with readability, including
bullet points, but bear in mind that asciidoctor only knows how to generate
groff(1) manpages. Some Linux distributions, and BSDs use mandoc(7) to render
man pages, and hence this isn't a superset of groff, so care should be taken
to not do anything too exotic.