[patch] remove obsolete error codes in documentation

[Dominique Pellé]

Hi

Attached patch removes obsolete error codes.
Those codes were removed from the code but
not from the documentation:

- E198 was removed in patch 8.1.0932
- E278 was removed in patch 8.2.0191
- E538 was removed in patch 8.1.2171
- E340 was removed in patch 8.1.1384

It also fixes a typo in runtime/doc/version6.txt

Regards
Dominique

[Bram Moolenaar]

Thanks, I'll include it.

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