Renamed description of error that might happen about invalid bounding indexes.
Renamed description of error that might happen about invalid bounding
indexes.

Introduced a description of error that always happens when bounding
indexes are invalid.

Introduced parameter descriptions for just a sequence and just a
sequence index.

Supplied docstrings for ELT and FILL.

clasp-developers/SICL,clasp-developers/SICL,vtomole/SICL,vtomole/SICL,clasp-developers/SICL,vtomole/SICL,vtomole/SICL,clasp-developers/SICL