More doc updates

More doc updates

* commands.texi (Reading One Event): Mention keyboard coding.

* functions.texi (Function Cells):
* eval.texi (Function Indirection): Update for the fact that
symbol-function no longer signals an error.

* keymaps.texi (Translation Keymaps, Translation Keymaps):
* nonascii.texi (Terminal I/O Encoding): Copyedits.

* data.c (Fsymbol_function): Doc fix.