Remove unused local vars
Consolidate arg length validation

First, we add arg length validation to CreateDocEntryForInstallMethod.

Next, we remove arg length validation from the various
Declare*WithDocumentation functions, as it is superfluous there: If the
arg list is too short, this simply triggers a GAP out of bounds error,
which should be sufficient for debugging. And beyond that, the
CreateDocEntry* functions already perform arg length validation.

This reduces code duplication, with all the usual benefits.
Reuse generated XML header across functions

The new global variable AUTODOC_XML_HEADER unifies five occurrences
of the same XML header (previously with tiny variations).
Use Directory() to handle path_to_xmlfiles

This way, CreateAutomaticDocumentation can now also accept paths
without trailing slash. In fact, it is now possible to directly pass
an IsDirectory object to it. Thus now this parameter matches the first
parameter of MakeGAPDocDoc.
Fix two bugs in PackageInfo.AutoDoc.TitlePage support

First, fix specifying a custom copyright string. Second, remove the
hardcoded message in the title comment saying that the "manual is
still under construction".