Add migration run time calculation to advanced search migration styleguide

Add info about how to calculate how long a migration will take to run to the Advanced Search migration style guide

The following discussion from !111678 (merged) should be addressed:

• @dgruzd started a discussion: (+6 comments)

  suggestion: Could I ask you to provide expected runtime for the backfill migration with these paramemters? Thanks!

We decided in the team meeting to also add information about dangers/not merging migrations late in the milestone. Also include considerations for long running migrations