Update the documentation content and formatting

[zverok]

This PR updates FileUtils docs. Initially, I just started to fix wrong example of usage of cp_r, which started with:

FileUtils.rm_r site_ruby + '/mylib', :force # doesn't work, should be force: true

...but then I reviewed the whole module docs and did the following changes:

• updated all code examples to use keyword args instead of hashrocket syntax;
• hidden some of internal constants and modules;
• made "See also" links linkable (see, for example, rm_rf -- it says "Read the documentation of rm_r first.", but rm_r isn't linked, because in docs source it should be documentation of rm_r or documentation of ::rm_r, not documentation of #rm_r);
• added a bit more explanations to some places (for example, currently introduction says "The options parameter is a hash of options, taken from the list :force, :noop, :preserve, and :verbose. :noop means that no changes are made. The other three are obvious." -- I believed that they are not :))
• (not sure about it) unhidden methods FileUtils::commands, FileUtils::options and several others -- considering they call-seq and docs, they've meant to be public.

PS: I believe it makes sense to change everywhere default value from verbose: nil to noop: false (for all keyword args), because nil for a boolean option intuitively reads like a "possible third option" (like "true is totally verbose, false is totally silent, nil is some reasonable defaults").