- 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"

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#### Description <!-- Describe your changes in detail --> Misuse of the exec module functions is a security concern. This PR updates the documentation to help Kamailio administrators use these functions securely. You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/3338

-- Commit Summary --

* exec: docs - added security warning

-- File Changes --

M src/modules/exec/doc/exec_admin.xml (24)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/3338.patch https://github.com/kamailio/kamailio/pull/3338.diff