Explain rationale for choosing bufread,read,write modules

[jongiddy]

Provide a clearer explanation of when to choose the different module implementations.

Make the write module example more specific to a problem that can only be handled using the write module.

Update inter-module links to RFC-1946 format.

Fixes #511

[Byron]

Thanks a lot - this looks like an improvement that does fix #511.

While I am here, I am merging this PR but invite @Zalathar to follow it up with further improvements in case it's not entirely fitting just yet.