Substring undocumented form

Hi all,

Someone submitted a comment to the documentation which made mention of
the fact that while we provide one form of the 3-parameter substring
function, we don't mention an alternative one.

substring(string from pattern for escape)
can be written:
substring(string, pattern, escape)

Curiously, this doesn't appear to be the same with similar functions.
For instance, the following can't be changed in a similar way:

overlay('Txxxxas' placing 'hom' from 2 for 4)
can't be written:
overlay('Txxxxas', 'hom', 2, 4)

position('om' in 'Thomas')
can't be written:
position('om', 'Thomas')

So this doesn't look like a general alternative form.  Is the
alternative form of substring deprecated or just missing from the
documentation?

Thanks

Thom Brown
Twitter: @darkixion
IRC (freenode): dark_ixion
Registered Linux user: #516935