Expand documentation of the SCVD format

[jkrech]

We need specification and documentation for:

• Which format specifier (%J, %M, %U, %E, %d, %x) supports structured access?
• What is the structured access syntax?
• What is the structured access behavior? (i.e. how does it work?)
• Examples with simple and complex usage together with the produced output

https://arm-software.github.io/CMSIS-View/latest/elem_member.html

[ReinhardKeil]

I believe these were experiential. Are these format specifiers used in some SCVD files? A potential candidate are the SCVD files of MDK Middleware. If not maybe these should be deprecated.

[KeilChris]

A quick check of the middleware pack showed that we use this for example in the Network.scvd file quite heavily.