Document the interaction of the "compilee" with the compiler's current module.

Document the interaction of the "compilee" with the compiler's current module.

This is a followup to 87c595c757b7db84ffdcfda96f736ab235e674a8 ("Compile
in a fresh module by default.") and
f65e2b1ec5ae1962e57322ac3085ab4d44025694 ("Honor and confine
expansion-time side-effects to `current-reader'.").

* doc/ref/api-evaluation.texi (Loading): Explain how to change
  `current-reader' in a compiler-friendly way.

* doc/ref/compiler.texi (The Scheme Compiler): Explain use of a fresh
  compilation module and separate `current-reader' fluid.

* test-suite/tests/compiler.test ("current-reader")["with eval-when"]:
  New test.