Specification of `sentry`

[ruben-arm]

The document doesn't seem to specify sentry, however it refers to the concept which in combination with __builtin_cheri_seal_entry suggests that it might be a part of the CHERI C/C++ model. Would it be possible to clarify this concept and associated required behaviours, please?

[nwf]

FWIW: there's a lot of prose in the ISA doc, section 3.8, some of which might be useful in the programming guide (and some of which is much more philosophical in nature and probably isn't useful here).

[rwatson]

A brief summary of “sentry capabilities” would be useful in the Background section on capabilities, perhaps entirely replacing the section on otypes given that they aren’t used for CHERI C/C++, but rather only for compartmentalisation and only on some CHERI architectures.