[mlir] Restrict dialect doc gen to a single dialect

[mlir] Restrict dialect doc gen to a single dialect

In the overwhelmingly majority of cases only one dialect is generated at a time
anyways, and this restriction more easily catches user error when multiple
dialects might be generated. We hit this semi-recently with the PDL dialect,
and circt+other downstream users are also actively hitting this as well.

Differential Revision: https://reviews.llvm.org/D125651