[TCLCORE] Documentation of the documentation command for 9.0

[Harald O. <har...@el...>]

Dear TCL team,

the encoding command was revised with TCL 9.

Many people contributed to this page. A rewrite is proposed by Nathan in 
the branch "encoding-for-review":

https://core.tcl-lang.org/tcl/info/4d6aa33b2f

Inspired from this change, a new version is proposed in the branch 
"encoding-for-review-alt":

https://core.tcl-lang.org/tcl/info/f5243d7263ce854a

Thank you for any comment and take care,
Harald