Improve migration guide

[mojito317]

Checklist

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

Description

Related to https://github.com/cloudant/nodejs-cloudant/issues/450 and https://github.com/IBM/cloudant-node-sdk/issues/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 281404bce3b728038f655d36a29ecebefe163902
• extended the line about plugins with the retry function 281404bce3b728038f655d36a29ecebefe163902 and 65cb790
• applied code style on the links for new operations 39fc65c2f12bfc94eb92c95cf27ec76891af8559
• fixed incorrect links (dde9461132a7a84039dc38af05e69a4dd744ef31):
  • 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 dde9461132a7a84039dc38af05e69a4dd744ef31
• Added a warning about the error handling c795cec9f74efd614077605819d6474a45fbef3e

Schema & API Changes

• "No change"

Security and Privacy

• "No change"

Testing

• N/A build or packaging only changes

Monitoring and Logging

• "No change"