Improve doc for hash tables

Improve doc for hash tables

* doc/lispref/hash.texi (Creating Hash, Defining Hash):
* src/fns.c (Fsxhash_eq, Fsxhash_eql, Fsxhash_equal):
Say that hashes are fixnums.
(Fmake_hash_table): Say that that an integer rehash-size
should be a fixnum.
* doc/lispref/hash.texi (Defining Hash): Say that hash and
comparison functions should be consistent and pure, and should
return quickly.