ETM: rename registers, doc tweaks

The register names are perversely not documented as zero-indexed,
so rename them to match that convention.  Also switch to lowercase
suffixes and infix numbering, matching ETB and EmbeddedICE usage.

Update docs to be a bit more accurate, especially regarding what
the "trigger" event can cause; and to split the issues into a few
more paragraphs, for clarity.

Make "configure" helptext point out that "oocd_trace" is prototype
hardware, not anything "real".

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>