BUG #13504: Types in math functions table is incorrect

The following bug has been logged on the website:

Bug reference:      13504
Logged by:          Tobias Pfeiffer
Email address:      tgpfeiffer@web.de
PostgreSQL version: 9.4.4
Operating system:   --
Description:

On http://www.postgresql.org/docs/9.4/static/functions-math.html in Table
9-3 there is a number of issues in the "Return Type" column.

For example:
- for ln it says "same as input", but the output is actually double
precision (both when checked with two eyes and `pg_typeof`)
- same for log
- for sign it says "same as input" but the output is actually numeric

I have not checked all of the functions in that table, but at least the ones
above need to be fixed, I think.