docs: fix information about prepared statement support

[TheOtherBrian1]

What kind of change does this PR introduce?

doc update

What is the current behavior?

Supavisor in transaction mode does not support standard prepared statements, but the documentation claims otherwise.

What is the new behavior?

Updated the FAQ

Additional context

The docs state:

As of 1.0 Supavisor supports prepared statements. Supavisor will detect
prepare statements and issue those to all database connections. All clients
will then be able to address those prepared statements by name when issuing
execute statements.

Prepared statements are not fully supported and this claim caused user confusion (ticket). It's worthwhile to add more context or remove the claim.

It's also worth re-opening up issue #69