Clarify ABI compatibility rules in MaterialX

It seems like the recently released version broke some ABI:
```
blender: symbol lookup error: /usr/lib/libusd_ms.so: undefined symbol: _ZTIN17MaterialX_v1_38_97ElementE
```

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This was first reported on https://gitlab.archlinux.org/archlinux/packaging/packages/blender/-/issues/17
CC @svenstaro

Start by reviewing the ABI failure shown in the issue and the linked Arch Linux packaging report. Trace which MaterialX release introduced the missing symbol and determine whether the compatibility rules or release behavior need correction. Done means the ABI break is explained and the applicable compatibility guidance or fix is agreed and documented.

Written by the indexing model from the issue text.