Update bsla_nonnullarg docs to note that BSLA_NONNULLARG indexing includes 'this' - DRQS 168976878 (#3645)
Update bsla_nonnullarg docs to note that BSLA_NONNULLARG indexing includes 'this' - DRQS 168976878 (#3645)

Minor documentation tweak. clang gives the following warning if you 
do `BSLA_NONNULLARG(1)`:

> '__nonnull__' attribute is invalid for the implicit this argument
bloomberg/bde,bloomberg/bde,bloomberg/bde,bloomberg/bde,bloomberg/bde