More GMP documentation

Add BigFloat and BigInt to stdlib docs, and hence helpdb.jl. Help is now available at the REPL. 

````jlcon
julia> help("BigFloat")
Loading help data...
Base.BigFloat(x)

   Create an arbitrary precision floating point number. "x" may be
   an "Integer", a "Float64", a "String" or a "BigInt". The
   usual mathematical operators are defined for this type, and results
   are promoted to a "BigFloat".

````

per #2111