Improve the usage of variable names in Scheme docstrings.

Improve the usage of variable names in Scheme docstrings.

* module/ice-9/boot-9.scm:
* module/ice-9/popen.scm:
* module/ice-9/pretty-print.scm:
* module/ice-9/r4rs.scm:
* module/rnrs/io/ports.scm:
* module/texinfo/string-utils.scm:
* module/web/http.scm:
* module/web/request.scm:
* module/web/response.scm:
* test-suite/vm/run-vm-tests.scm: Make the variable names in Scheme docstrings more
  consistent.  Replace a few instances of @var with @code when appropriate.