rtpengine: update doc

[igorolhovskiy]

• explicit mention of label= flag
• add explanations regarding mos_A/B_label_pv parameters
• typo fixes

Pre-Submission Checklist

• [X] Commit message has the format required by CONTRIBUTING guide
• [X] Commits are split per component (core, individual modules, libs, utils, ...)
• [X] Each component has a single commit (if not, squash them into one commit)
• [X] No commits to README files for modules (changes must be done to docbook files in doc/ subfolder, the README file is autogenerated)

Type Of Change

• [X] Small bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds new functionality)
• [ ] Breaking change (fix or feature that would change existing functionality)

Checklist:

• [ ] PR should be backported to stable branches
• [X] Tested changes locally
• [ ] Related to issue #XXXX (replace XXXX with an open issue number)

Description

Some clarifications regarding collecting MOS stats via rtpengine.