Update GetForeignPlan documentation.
Update GetForeignPlan documentation.

Commit 385f337c9f39b21dca96ca4770552a10a6d5af24 added a new argument
to the FDW GetForeignPlan method, but failed to update the documentation
to match.

Etsuro Fujita

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl