Document that date math is locale independent
Document that date math is locale independent

With this commit we add a note to the API conventions documentation that
all date math expressions are resolved independently of any locale. This
behavior might be puzzling to users that try to specify a different
calendar than a Gregorian calendar.

Closes #37330 
Relates #37663
strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra