Merged some lost updates in 1b13aa-bc4487.

- XML-equivalent examples added for alternative communition.
- DNS SRV lookup: SHOULD->MUST, RECOMMENDED->SHOULD
- default alignment: bit-packed
- The section for recommended option is removed
- Added preprocessing and postprocessing section at the top of implementation notes
- Some discussions ([YD] FIXME in comments) added.
- Beginning of sessionWideBuffer is editted to clarify this is not an EXI option.
removed wrong examples for communication restart.
Recovered default schema section in implementation note. THIS IS STRONGLY REQUIRED FOR ALTERNATIVE BINDING.

schemaId for default schema is replaced undef urn:xmpp:exi:cs:default
removed unneccessary (agreed) implementation notes on schemaId.
Added(recoverd) shortcut setup for alternative binding using schemaId.