Document that we support srfi-46 and add it to %cond-expand-features.

Document that we support srfi-46 and add it to %cond-expand-features.

* module/ice-9/boot-9.scm (%cond-expand-features): Add srfi-46.

* doc/ref/srfi-modules.texi (SRFI-0): Add srfi-46 to the list of core
  features.
  (SRFI-46): New node.

* doc/ref/api-macros.texi (Syntax Rules): Mention that the custom
  ellipsis identifier support is specified by SRFI-46.

* test-suite/tests/syntax.test ("syntax-rules"): Add ellipsis hygiene
  test from SRFI-46.