Add more docs about :already_started for DynamicSupervisor (#14298) #11678

Job	Run time
Windows Server 2019, Erlang/OTP 26.2	6m 8s
Ubuntu 24.04, Erlang/OTP 27.1 (deterministic)	5m 56s
Windows Server 2019, Erlang/OTP 27.1	6m 52s
Ubuntu 24.04, Erlang/OTP 27.1	5m 34s
Ubuntu 24.04, Erlang/OTP 27.0	5m 0s
Check POSIX-compliant	11s
Ubuntu 24.04, Erlang/OTP 26.0	4m 43s
Check Licence Compliance	8m 28s
Ubuntu 24.04, Erlang/OTP master	1m 42s
Ubuntu 24.04, Erlang/OTP maint	4m 45s
	49m 19s

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