More error checks in am1

[Bill E]

As a result of discussions with Mike G, am1 now enforces some memory checks. These are fairly easy mistakes to make and macro1 doesn't check for them.

If code goes past the end of its bank, i.e. past address 07777, an error is given. Macro would just silently wrap back around to 0, letting you wipe out code there.

If code would overwrite code already at a memory location, an error is given. Macro didn't have any check like this at all.

Docs are also updated.

Bill