doc: small fixes in the buffer.md

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, buffer

Description of change

1. var => const / let.
2. Fix a syntax error.
3. Fix wrong arguments in function calls*.
4. Fix some copy-paste artifacts.
5. Repeat the remark about buffer module property where it is needed.
6. Add a missing variable declaration.
7. Remove / fix wrong remarks in the buffer.md
8. Fix examples in buffer.md to avoid confusion (fixes #9786 (closed))
9. Remove an extraneous word in the buffer.md

*As to null => undefined in point 3, it is a temporary fix untill #9801 (closed) is addressed.