The P4 env vars are described as P4 Plugin.blurb.

Example: P4_PORT : P4 Plugin.blurb

Click on a job -> Configure

[scroll to ‘Build’ -> ‘Execute [ Windows Batch | Shell ] Command’]

Under that box is a link to ‘See the list of available environment variables’

If you click that, you get help box/tab saying:

-------------------------------------------
The following variables are available to shell scripts
[snip]

JOB_URL
Full URL of this job, like http://server:port/jenkins/job/foo/ (Jenkins URL must be set)

P4_CHANGELIST
P4 Plugin.blurb

P4_CLIENT
P4 Plugin.blurb

P4_PORT
P4 Plugin.blurb

P4_USER
P4 Plugin.blurb

P4_TICKET

P4 Plugin.blurb