Correct the handling of symbols with position in equal

Correct the handling of symbols with position in equal

* src/fns.c (internal_equal): Only regard symbols with position
as their symbols when symbols-with-pos-enabled is non-nil.

* doc/lispref/symbols.texi (Symbols with Position): Expand the
description of symbols with position, in particular the way
they work with eq and equal.

* doc/lispref/objects.texi (Equality Predicates): Describe how
eq and equal handle symbols with position.

* test/src/fns-tests.el (fns-tests-equal-symbols-with-position):
New tests for symbols with position.