This is very common in uml, but you are correct, it is not a part of the rational/omg specification (great place to learn the basics, but the spec is only a starting point IMHO).
UML predates standards (originally it was done on cocktail napkins, with swizzle sticks, and plastic olive swords, etc...)
Common usages in the wild:
A #-- B: a back reference (the useful relationship is A to B, but is held in B, not A)
A x-- B: a non relationship... IE: B is not related to A. Common where a relationship occurs in parent classes, but are not useful in certain children
A }-- B: See entity relationship diagrams (there are more relation endings listed there)
A +-- B: Required relationship (IE: non-null) If you look at Deployment Diagrams... A o-- B is commonly used to denote optional relationships (technically no end marker does not denote optinality, but this is rather pedantic, so this kind of thing is uncommon in the wild).
Listed, but not a part of UML proper:
A <-- B: directional relationship (IE: B is related to A, but A is not related to B).