Improve migration guide

[mojito317]

Checklist

• Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• Added tests for code changes or test/build only changes
• Updated the change log file (CHANGES.md|CHANGELOG.md) or test/build only changes
• Completed the PR template below:

Description

Related to #450 and IBM/cloudant-node-sdk#410

My goal was to improve the migration guide dispersing the fog around the support of the retry feature ane error handling.

Approach

• removed false sentence about the retry function 281404b
• extended the line about plugins with the retry function 281404b and 65cb790
• applied code style on the links for new operations 39fc65c
• fixed incorrect links (dde9461):
  • postReplicate was excluded from new SDKs, so I pointed replicate()/replicateDb() to the putReplicationDocument operation
  • link for postChanges changed
• for old AsStream functions I mentioned the new AsStream operation if available dde9461
• Added a warning about the error handling c795cec

Schema & API Changes

• "No change"

Security and Privacy

• "No change"

Testing

• N/A build or packaging only changes

Monitoring and Logging

• "No change"

[emlaver]

Looks great!