docs: fix handling of the old tool in the M6 description

The description before this commit was wrong: in random-toolchanger
machines, tools don't "own" a slot in the tool changer.

The new words are more vague, and apply correctly to both random-tc and
nonrandom-tc machines.
docs: remove incorrect T-word info in M6 description

The T-word specifies the tool number (ie the tool id), *not* the slot
number (ie not the toolchanger magazine pocket id).

But in any case, surely the T-word is adequatly described in the section
describing the T-word, and doesn't need a repeat in the M6 section.
And the T-word section is linked from the M6 section, so i think we're
fine here.
docs: clarify numbered parameters

Numbered parameters describe other things in addition to the loaded
tool...
canon: fix some comments

The CHANGE_TOOL_NUMBER() canon call is *only* called by "M61 Qn".

The SELECT_POCKET() canon call is *only* called by "Tn".

The CHANGE_TOOL() canon ccall is *only* called by "M6".
canon: rename a CHANGE_TOOL_NUMBER() argument for clarity

Naming a variable "number" isn't very helpful, because we have "tool
numbers" and "pocket numbers" in the tool changer code.  Call this
variable "pocket_number" instead, to disambiguate.