doc,test: mem protection must be observed in ffi

[bengl]

When using ffi.toBuffer, memory protection on any memory pages exposed must be observed by the caller, otherwise crashes will occur.

Now documented, and tested.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.54%. Comparing base (3f52482) to head (43f68d7).
⚠️ Report is 35 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62818      +/-   ##
==========================================
- Coverage   91.55%   89.54%   -2.02%     
==========================================
  Files         356      706     +350     
  Lines      149601   218222   +68621     
  Branches    23395    41686   +18291     
==========================================
+ Hits       136967   195398   +58431     
- Misses      12371    14739    +2368     
- Partials      263     8085    +7822     

see 505 files with indirect coverage changes

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