Docs: Incorrect info in architecture diagram

[andypugh]

https://github.com/LinuxCNC/linuxcnc/blob/master/docs/src/code/LinuxCNC-motion-controller.png

Suggests that the HAL code is in "EXTINTF.C", and it's not, it's in various hal*.* files.

[grandixximo]

@andypugh modified the picture, PR ready.

[Zia-research]

Disclaimer. AI-generated work, directed and published by me; every claim below carries
a commit or file:line citation.

A fix for this already exists, and it is closed on a condition that will not be met.
#3967 corrected the caption in both PNG variants and said "Closes #3843". @smoe closed it
on 2026-04-26 with "Closed upon acceptance of #3718". #3718 was never accepted, and on
2026-08-05 its own author proposed closing it. So #3967 is waiting on something that is
not coming — reopening it would close this issue.

Dating the error, since it is checkable. extintf.h was deleted on 2005-11-05
(7ed547ea17, "removed obsolete hal_intf files"), and the diagram was committed on
2012-11-19 (b60c20198e). The caption was already seven years stale on the day the image
was drawn. HAL today is src/hal/hal_lib.c, hal_lib_extra.c and hal_lib_query.c.

One caveat for whoever picks this up. This issue links
LinuxCNC-motion-controller.png, which nothing in the repository references —
code-notes.adoc:154 renders the -small variant. #3967 correctly updated both; a
narrower fix touching only the linked file would change nothing a reader sees.

And the caption is not the only error in that figure. I audited it against master and
found eleven, of which this is one; @zz912 gestured at another in #3967, joints versus axes.
Details at https://github.com/Zia-research/linuxcnc-audit if useful. I am not proposing to
take the figure on — only flagging that closing this issue would not make it correct.