[MINOR][ML][PYTHON][DOC] Remove use of JavaMLWriter/Reader in public Python API docs

## What changes were proposed in this pull request?

Removed instances of JavaMLWriter, JavaMLReader appearing in public Python API docs

## How was this patch tested?

n/a

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #12542 from jkbradley/javamlwriter-doc.