exec: docs - added security warning

[sandrogauci]

• Added warning about potential for OS Command Injection
• Updated invalid examples

previous example gives the following error:
pv_parse_spec2(): error searching pvar "rU.txt"

Pre-Submission Checklist

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• 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

• 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
• Tested changes locally
• Related to issue #XXXX (replace XXXX with an open issue number)

Description

Misuse of the exec module functions is a security concern. This PR updates the documentation to help Kamailio administrators use these functions securely.

[miconda]

Thanks!

[Jonnypleasehelp]

Thanks

[Jonnypleasehelp]

Yes I am learning as we go along it's all new language for me