Docs: Add PartialSortExec documentation (apache#23092)

@mhilton mentioned the other day that he was looking for PartialSortExec
but it wasn't well documented

So let's fix that!

## Summary
- expand PartialSortExec rustdoc with a concise explanation
- add an ASCII example showing partial sorting from (a, b) to (a, b, c)

cc @berkaysynnada and @akurmustafa perhaps you might have some time to
review this PR

Author: alamb

datafusion/physical-plan/src/sorts/partial_sort.rs

@@ -77,7 +77,37 @@ use datafusion_physical_expr::LexOrdering;
 use futures::{Stream, StreamExt, ready};
 use log::trace;
 
-/// Partial Sort execution plan.
+/// Sort execution plan for inputs that are already partially sorted.
+///
+/// This operator takes input ordered by a prefix of the required ordering, and
+/// produces output ordered by the required ordering. This is useful for
+/// unbounded or large inputs where a [`SortExec`] must buffer all rows before
+/// producing any output.
+///
+/// [`PartialSortExec`] relies on the property that rows with the same sort
+/// prefix are contiguous, so it can sort one prefix group at a time, emitting
+/// completed groups without reading (and buffering) the entire input.
+///
+/// For example, if the required output is `(a, b, c)`, but the input is only
+/// ordered by `(a, b)`, `PartialSortExec` sorts only within each `(a, b)`
+/// group to produce output ordered by `(a, b, c)`.
+///
+/// ```text
+/// input ordered by a, b              output ordered by a, b, c
+///
+/// +---+---+---+                      +---+---+---+
+/// | a | b | c |                      | a | b | c |
+/// +---+---+---+                      +---+---+---+
+/// | 0 | 0 | 3 |  -- same group  -->  | 0 | 0 | 2 |
+/// | 0 | 0 | 2 |                      | 0 | 0 | 3 |
+/// | 0 | 1 | 1 |  -- single row  -->  | 0 | 1 | 1 |
+/// | 0 | 2 | 4 |  -- same group -->   | 0 | 2 | 0 |
+/// | 0 | 2 | 0 |                      | 0 | 2 | 4 |
+/// | 1 | 0 | 5 |  -- single row  -->  | 1 | 0 | 5 |
+/// +---+---+---+                      +---+---+---+
+/// ```
+///
+/// [`SortExec`]: crate::sorts::sort::SortExec
 #[derive(Debug, Clone)]
 pub struct PartialSortExec {
     /// Input schema