- Use description found in https://sip-router.org/wiki/ref_manual/timers

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#### Checklist: <!-- Go over all points below, and after creating the PR, tick the checkboxes that apply --> - [x] PR should be backported to stable branches - [ ] Tested changes locally - [x] Related to issue #3939

#### Description <!-- Describe your changes in detail -->

Fix documentation according to the description found in https://sip-router.org/wiki/ref_manual/timers. They better reflect the actual procedure. You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/4207

-- Commit Summary --

* tm: Update docs of fr_timer to match the implementation

-- File Changes --

M src/modules/tm/doc/params.xml (8)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/4207.patch https://github.com/kamailio/kamailio/pull/4207.diff