Clarify uses of "probabilistic"

Not all the weighting schemes we now support are probabilistic.

glossary.rst

@@ -32,8 +32,8 @@ Retrieval, while others have a specific meaning in the context of Xapian.
  list of terms joined with a combination of operators such as AND, OR,
  AND_NOT).  In many systems, these documents are not ranked according to their
  relevance.  In Xapian, a pure Boolean query may be used, or alternatively a
- Boolean style query can filter the retrieved documents, which are then ordered
- using a probabilistic ranking.
+ Boolean style query can filter the retrieved documents, which are then ranked
+ using a weighting formula.
 
 **Brass**
  Brass was the current "under development" database format in Xapian 1.2.x,
@@ -157,9 +157,10 @@ Retrieval, while others have a specific meaning in the context of Xapian.
  returned.
 
 **Probabilistic IR**
- Probabilistic IR is retrieval based on probability theory, this can produce a
- ranked list of documents based upon relevance.  Xapian uses probabilistic
- methods (the only exception is when a pure Boolean query is chosen)
+ Probabilistic IR is retrieval using a weighting formula derived from
+ probability theory to produce a ranked list of documents based upon estimated
+ relevance.  Xapian supports several families of weighting schemes, some of
+ which are based on probabilistic methods.
 
 **Quartz**
  Quartz was the database format used by Xapian prior to version 1.0.  Support