Make more documentation available through `:doc`

[gallais]

• :doc for keywords ([ new ] :doc for keywords #2028)

is a first step but ideally I'd like to be able to access at least a basic blurb about
pretty much any aspect of the language from the REPL using :doc.

In that spirit we should have:

• :doc for reserved symbols (reservedSymbols in Parser.Lexer.Source)
• :doc for pragmas (todo: add validPragmas to Parser.Lexer.Source)
• :doc % for a list of pragmas

[gallais]

We can piggy back on #1705