Fix formatting of long function description in chew output

Fix formatting of long function description in chew output

Currently, if a function description spans a line, the resulting info
can look like this:

 -- Function: long bfd_canonicalize_reloc
     (bfd *abfd, asection *sec, arelent **loc, asymbol **syms); Call the
     back end associated with the open BFD ABFD and translate the
     external form of the relocation information attached to SEC into
     the internal canonical form.  Place the table into memory at LOC,

That is, the function prototype runs together with the text in an ugly
way.  This patch fixes this by introducing a new primitive, so that
the generated Texinfo can be a bit nicer.  Now this output looks like:

 -- Function: long bfd_canonicalize_reloc (bfd *abfd, asection *sec,
          arelent **loc, asymbol **syms);
     Call the back end associated with the open BFD ABFD and translate
     the external form of the relocation information attached to SEC

2023-02-17  Tom Tromey  <tom@tromey.com>

	* doc/doc.str (SYNOPSIS): Use collapse_whitespace.
	* doc/chew.c (collapse_whitespace): New function.
	(main): Register collapse_whitespace.