[spydon]

What kind of change does this PR introduce?

docs update

What is the current behavior?

Following up on #23, the rest of the limitations list had not been audited against the code. I verified every bullet empirically by driving the mock client through a SupabaseClient, and found one stale entry plus two wording issues.

• count and head requests are not supported. is stale. _handleHead and the count= Prefer header handling make both work, and there is already a count test group covering it. A HEAD-only .count(CountOption.exact) returns the right number.
• Respect nullsFirst on ordering is not supported. is accurate but undersells it. The order parameter arrives as column.asc.nullslast, and the mock only reads the first two segments, so nullsFirst is ignored. Worse, ordering by a column containing null throws type 'Null' is not a subtype of type 'num' of 'other' rather than sorting.
• The serial comma in the auth/realtime/storage bullet was dropped in Update limitations in documentation #23.

What is the new behavior?

• Removed the count/head limitation and added a matching bullet to the features list, since that support was otherwise undocumented.
• Reworded the nullsFirst bullet to say it is ignored and that a nullable column throws.
• Restored the serial comma.

I re-verified that the remaining bullets are all still true, so they are untouched:

Limitation	Verified behavior
Nested referenced tables	still unsupported
!inner join	the !inner branch is an empty no-op, so the row is returned with the referenced table set to null instead of being excluded
Renaming column names	select('headline:title') returns [{}]
aggregate functions	select('score.sum()') returns [{}, {}]
auth, realtime, storage	no handling anywhere in the client

Docs only, no code change. dart analyze is clean and all 90 tests pass.