How is a procedure's signature documented?

[Diogo F. S. Ramos]

Is there a pattern they must/should follow?

Using (ice-9 documentation), I've been looking at some procedure
signatures.  For what I can tell, they are described in the first line
of `object-documentation'.

Here are some examples:

(object-documentation car)
 => "- Scheme Procedure: car "

(object-documentation cdr)
 => "- Scheme Procedure: cdr "

(object-documentation list)
 => "- Scheme Procedure: list .  objs"

(object-documentation array-ref)
 => "- Scheme Procedure: array-ref v [idx0 [idx1 .  idxN]]"

`car' and `cdr' don't show a lambda list.  `list' shows a dot, which I
assume it's an indication that the following parameter is a rest.
`array-ref' too has a dot, but it also has []s which IIUC represent
optional arguments but are not valid lambda list constituents.