rename Ho_Net.empty_net to Ho_Net.empty

All similar structures (e.g. Net and Raw) use "empty" not "empty_net";
it seems right to follow a single naming convention.
use "fn" rather than "\" in SML code samples
pair: with T1 encoding one can search for names with underscodes in PDF
pair: export pvariant function, as it is already listed in manual
guide: grammar fix
trailing newlines in *.{sml,sig} files from src/ removed

Trailing newlines from SML files in src/ were rendered in HTML documentation.
whitespace cleanup: consistent use of tabs and spaces
first_tok: error message improved; unused code removed
correct indent (3 spaces) of documentation in .sig files
doc: full stop at the end of SYNOPSIS sections
help: rewrite as parallel structures (firstly, secondly, thirdly)
Prim_rec: whitespace cleanup in both .sig and .doc
SML whitespace cleanup
Merge pull request #209 from ptroja/master

Minor cleanups for SML, TeX and .doc files