gas/doc: clarify internal symbol vs local symbol terminology

In ELF, "local symbols" refer to symbols of STB_LOCAL binding. The
doc is inconsistent: while it uses "local symbol" in places like .local,
"local symbols" are also used for .L-prefixed symbols (as determined by
bfd_is_local_label).

Rename "Local Symbol Names" to "Internal Symbol Names" for .L-prefixed
symbols, and rename "Local Labels" to "Numeric Local Labels" for
N:/Nb/Nf numeric labels.  This avoids confusion with ELF STB_LOCAL
"local symbols".

The term "internal symbol" is chosen over alternatives like "temporary
symbol" because it describes the purpose of these symbols: they are for
internal use by compilers and assemblers, not meant to be visible
externally (albeit they can become visible in certain situations).
While ELF defines STV_INTERNAL as a symbol visibility, it is rarely used
in practice and unlikely to cause confusion (only used by SGI for their
link-time interprocedural optimization; useless on other OSes).