Add gem.deps.rb documentation for `gem install`
Add `gem help gem_dependencies` document
Fix word wrapping of `gem help install`
Document GemDependencyAPI#gem

Fixup `gem help gem_dependencies` bugs.
Document GemDependencyAPI#git
Document GemDependencyAPI#gemspec
Document GemDependencyAPI#group
Document GemDependencyAPI#platform
Document GemDependencyAPI#platforms
GemDependencyAPI#ruby now supports engine
Document GemDependencyAPI#source