doc: add LDoc Type section for callback function signatures.

* lib/std.lua.in (normalizecb): Document signature of memoize
callback function.
(memoize): Set type of *normalize* argument to new `normalizecb`
signature.
* lib/std/functional.lua (predicate): Document signature for a
predicate function.
(filter): Set type of *p* argument to new `predicate` signature.
* lib/std/io.lua (fileprocessor): Document signature of
process_files callback function.
(process_files): Set type of *fn* to new `fileprocessor` function.
* lib/std/string.lua (opentablecb, closetablecb, elementcb)
(paircb, separatorcb): Document signature for render callback
functions.
(render): Set type of callback functions accordingly.
* lib/std/table.lua (comparator): Document signature of sort
comparison function.
(sort): Set type of *c* argument to new `comparator` signature.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>