Clarify evmc_message::code_address usage

[chfast]

• Clarifies the documentation around evmc_message::code_address and other evmc_message fields.
• Fixes the usage of code_address vs destination in the "precompiles vm" example.
• Resolves the issue reported in Add code_address to evmc_message #611 (comment).

[codecov]

Codecov Report

Merging #615 (ab5a01a) into master (54bf895) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #615   +/-   ##
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  Coverage   95.73%   95.73%           
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  Files          24       24           
  Lines        3987     3987           
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  Hits         3817     3817           
  Misses        170      170           

[chfast]

@jlokier, please check if this clarifies the situation around precompiles implementation.

[yperbasis]

It also makes sense to change the description of EVMC_CAPABILITY_PRECOMPILES to say
"... for the range of code addresses" instead of "... for the range of destination addresses".