`HB.about` should point to `HB.howto` and conversly

[CohenCyril]

These two commands should give pointers to one another e.g.

• HB.about C, where C is a class, should say

C is a class, the associated type is `ctype` (long name `C.type`).
You can use `HB.howto ctype` to know which factories are available to instanciate it.
...

• HB.howto C.type should say

...
Use ` HB.about X.Build` to know how to build a factory `X` listed above