doc: manual: add text on the interaction of extrasymbols and dummy indices #720

Job	Run time
Build tarball distribution	54s
Build PDF document	1m 49s
Build form on ubuntu-24.04	6m 52s
Build tform on ubuntu-24.04	6m 49s
Build form on macos-15-intel	23m 19s
Build tform on macos-15-intel	21m 16s
Build tform on windows-2022	20m 45s
Build form on macos-14	5m 8s
Build HTML document	1m 59s
Build form on windows-2022	21m 8s
Build form on ubuntu-24.04-arm	5m 7s
Build tform on macos-14	4m 48s
Build tform on ubuntu-24.04-arm	6m 1s
Publish binaries to GitHub Releases	1m 20s
	2h 7m 15s

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