That behavior isn't usually explicitly documented but it is a consequence of the tokenizing and production definitions. IOW, extremely likely that all Pascal compilers (that support selection using a ".") support that construction.
It's not just whitespace but any "null token", for instance:
type
ARecord = record
Field1 : integer;
end;
var
v : ARecord;
begin
v { a comment before the field selector } . { another comment } Field1 := { comment } 1 { comment }; { comment }
end.
You can have block comments where whitespace is allowed. The same thing can be done with other constructions, e.g., "array1 { comment } [ { comment } 1 { comment } ] := ...". IOW, any token that is a delimiter is also a separator therefore a { comment } can usually (probably always} precede or follow it.
It's documented in the way Pascal is tokenized and by the structure common to all its productions. This is part of what makes Pascal a free form language (unlike, for instance, Python). Same thing can be done in C/C++ and other free form languages, again for the same reason.