[modbus] Document modbus register write_lambda

[jpeletier]

Description:

Related issue (if applicable): fixes

Pull request in esphome with YAML changes (if applicable):

• esphome/esphome#<esphome PR number goes here>

Checklist:

• [x] I am merging into next because this is new documentation that has a matching pull-request in esphome as linked above.
  or

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