Add documentation for txCounter #6921

Job	Run time
bittide-instances synthesis matrix generation	33s
bittide-instances hardware-in-the-loop test matrix generation	33s
Build dependencies	3m 46s
license-check	10s
Basic linting	1m 26s
Rust Lints	38s
bittide-experiments unittests	1m 42s
bittide-instances unittests	1m 36s
bittide-instances doctests	1m 33s
Bittide tests	9m 30s
Firmware Limit Checks	1m 1s
Firmware Support Unit Tests	2m 35s
synth (safeDffSynchronizer, hdl)	1m 23s
synth (swCcOneTopologyTest, test)	20m 23s
synth (linkConfigurationTest, test)	16m 25s
synth (transceiversUpTest, test)	16m 6s
synth (vexRiscvTcpTest, test)	10m 12s
synth (vexRiscvTest, test)	8m 59s
HITL (swCcOneTopologyTest, test)	6m 34s
HITL (linkConfigurationTest, test)	1m 42s
HITL (transceiversUpTest, test)	2m 20s
HITL (vexRiscvTcpTest, test)	1m 50s
HITL (vexRiscvTest, test)	1m 23s
Generate clock control report	0s
All jobs finished	3s
	1h 52m 23s

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