add support for existing param `process.maxSecondsUntilRampdown.input`

[belforte]

please add this param to the documentation and add a unit test for it.
It is used in CRAB automatic splitting algorithm to refine splitting by lumi (which is very
difficult to configure with just one number of lumi per job) to to make jobs target
a sensible running time (8h) and terminate gently even if they need to run longer
(so to avoid hitting condor wall time limit). CRAB will then incrementally add more shorter jobs
to process the leftover lumis.

reference: e5abaaa

[cmsbuild]

cms-bot internal usage

[cmsbuild]

A new Issue was created by @belforte.

@Dr15Jones, @ftenchini, @makortel, @mandrenguyen, @sextonkennedy, @smuzaffar can you please review it and eventually sign/assign? Thanks.

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[makortel]

assign core

[cmsbuild]

New categories assigned: core

@Dr15Jones,@makortel,@smuzaffar you have been requested to review this Pull request/Issue and eventually sign? Thanks