Add clarification for GCRS positions

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Coverage Report

File	Stmts	Miss	Cover	Missing
api/core
propagation_strategies.py	79	61	23%	81–186, 443–449, 452
utils.py	134	32	76%	38, 74–81, 105–116, 160–172, 212–216, 464–465, 479–480, 489, 682–683
api/core/database
models.py	49	12	76%	31–33, 36, 63–69, 72
satellite_access.py	32	23	28%	11–22, 26–39, 44–75
tle_access.py	55	18	67%	70–87, 111, 126, 138, 149–150, 157–159
api/core/versions/v1
routes.py	196	64	67%	42, 83, 134–146, 154–156, 221–222, 321, 407–408, 507–508, 591–592, 685–686, 727–749, 771–794, 834–883
tasks.py	51	14	73%	281–301, 359, 380–383
api/utils
coordinate_systems.py	36	28	22%	24–35, 55–70, 86–98, 117–131, 153–174
TOTAL	701	252	64%

Tests	Skipped	Failures	Errors	Time
33	0 :zzz:	0 :x:	0 :fire:	35.183s :stopwatch: