docs: explain default session expiration #33219

Job	Run time
collect-and-verify	2m 56s
lms-1(py=3.11,dj=pinned,mongo=7.0)	8m 45s
lms-2(py=3.11,dj=pinned,mongo=7.0)	11m 8s
lms-3(py=3.11,dj=pinned,mongo=7.0)	14m 1s
lms-4(py=3.11,dj=pinned,mongo=7.0)	19m 32s
lms-5(py=3.11,dj=pinned,mongo=7.0)	9m 40s
lms-6(py=3.11,dj=pinned,mongo=7.0)	15m 40s
openedx-1-with-lms(py=3.11,dj=pinned,mongo=7.0)	9m 49s
openedx-2-with-lms(py=3.11,dj=pinned,mongo=7.0)	14m 37s
openedx-1-with-cms(py=3.11,dj=pinned,mongo=7.0)	7m 41s
openedx-2-with-cms(py=3.11,dj=pinned,mongo=7.0)	5m 26s
cms-1(py=3.11,dj=pinned,mongo=7.0)	3m 18s
cms-2(py=3.11,dj=pinned,mongo=7.0)	11m 21s
common-with-lms(py=3.11,dj=pinned,mongo=7.0)	7m 46s
common-with-cms(py=3.11,dj=pinned,mongo=7.0)	3m 59s
xmodule-with-lms(py=3.11,dj=pinned,mongo=7.0)	7m 21s
xmodule-with-cms(py=3.11,dj=pinned,mongo=7.0)	7m 13s
xmodule-with-cms(py=3.11,dj=pinned,mongo=7.0)	7m 38s
xmodule-with-lms(py=3.11,dj=pinned,mongo=7.0)	7m 39s
Unit tests successful	2s
coverage (3.11)	1m 42s
compile-warnings-report	12s
merge-artifacts	8s
	2h 57m 34s

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