excise use of "iff" in the manual

excise use of "iff" in the manual

* doc/ref/api-compound.texi:
* doc/ref/api-control.texi:
* doc/ref/api-data.texi:
* doc/ref/api-macros.texi:
* doc/ref/api-modules.texi:
* doc/ref/api-procedures.texi:
* doc/ref/api-scheduling.texi:
* doc/ref/api-smobs.texi:
* doc/ref/api-undocumented.texi:
* doc/ref/api-utility.texi:
* doc/ref/compiler.texi:
* doc/ref/intro.texi:
* doc/ref/scheme-using.texi:
* doc/ref/sxml.texi:
* doc/ref/web.texi: Change uses of "iff" to "if, otherwise".  Fixes bug
  10302.