Don't use negative positions for docstrings

Don't use negative positions for docstrings

These were used for user-variables (doc starts with a '*'), but we don't
use that convention any more.  On my machine, there are no docstrings
starting with a '*' in etc/DOC.

* src/doc.c (Fsnarf_documentation): Never use negative positions.
(get_doc_string): Don't use eabs; position must now be positive, and all
callers are verified to do that.

(cherry picked from commit c7f6da7a4141a9f3492b8dbb0c1de710bd4757fa)