Add documentation on using multiple filters in summary endpoints

[erika-tyagi]

Using an ampersand delimiter works for adding multiple filters to summary endpoint requests:

educationdata using "school ccd enrollment", summaries(sum enrollment using fips race) sub(year=2018&fips=11)

But it would be ideal to (at least) add an example of doing that in the documentation and (ideally) make that syntax easier to use (e.g. comma separated, rather than & separated).