Documentation for SubstructLibrary.get_matches should be explicit that the default number of max matches is 1000

**Describe the bug**
Without specifying max_matches, it is 1000. This is due 
```
static constexpr int d_defaultMaxResults = 1000;
```
https://github.com/rdkit/rdkit/blob/5755fddb9e037a526cb8f8578e1933da18e3ccb9/Code/MinimalLib/minilib.h#L351

**Expected behavior**
When max_matches is unspecified, all the matches are returned.
