Document modelling of Memory-mapped I/O

[tautschnig]

Add documentation of marking memory regions as valid memory as well as read/write emulation functions. The documentation is based on our boot code paper (Model checking boot code from AWS data centers, Sections 4.1 and 4.2).

Fixes: #6997

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Base: 77.88% // Head: 77.88% // No change to project coverage 👍

Coverage data is based on head (845d238) compared to base (c764c1d).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7157   +/-   ##
========================================
  Coverage    77.88%   77.88%           
========================================
  Files         1576     1576           
  Lines       181856   181856           
========================================
  Hits        141645   141645           
  Misses       40211    40211           

Impacted Files	Coverage Δ
src/util/expr.h	97.02% <ø> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.