Work around for #18192 such that the documentation builds properly when -Infinity is used as default argument.

Also replace

if name not in firstline:
    ...
signature = firstline.split(name, 1)[-1]
if signature.startswith("(") and signature.endswith(")"):
    ...

by a single regexp search.

CC: @simon-king-jena @mwageringel

Component: documentation

Author: Jeroen Demeyer

Branch/Commit: u/jdemeyer/ticket/19100 @ 9ba5f19

Issue created by migration from https://trac.sagemath.org/ticket/19100