[CLI] xdcr-replicate man page - migration mode and explicit rule based mapping line needs to be removed

There is a line in xdcr-replicate man page regarding migration mode and explicit rule-based mapping:

Note that when migration mode is enabled Explicit Rule-Based Mapping is used instead. The Rule-Based Mapping expressions have the same format as the filtering expressions in advance filtering.

Discussed this with the XDCR team and looks like this is not true. Explicit mapping rules follow the same format for migration mode as well. Please remove this line from man page.

Also, we noticed that there is no example for setting the migration mode - it will be good to have one - Pavithra Mahamani can help with the example if you plan to do this