Remove `master_modify_multiple_shards` from the docs

[onderkalaci]

Citus supports modification queries through SQL interface. I think we don't need (and should not) have master_modify_multiple_shards on the docs. The UDF has various limitations, and no single advantage over regular SQL commands.

[jonels-msft]

Just letting you know I noticed this issue was created. Not sure how high the priority is vs some other things, but we'll consider it next time we do prioritization for community docs.

[marcocitus]

We will likely give a deprecation warning when calling this function starting from the next release.