[lldb][progress][NFC] Clarify Doxygen comments for details field (llvm#86002)

The Doxygen comments for the `details` field of a progress report
currently does not specify that this field will act as the initial set
of details for a progress report that gets updated with
`Progress::Increment()`. This commit clarifies this.

Author: chelcassanova

lldb/include/lldb/Core/Progress.h

@@ -66,7 +66,11 @@ class Progress {
   /// @param [in] title The title of this progress activity.
   ///
   /// @param [in] details Specific information about what the progress report
-  /// is currently working on.
+  /// is currently working on. Although not required, if the progress report is
+  /// updated with Progress::Increment() then this field will be overwritten
+  /// with the new set of details passed into that function, and the details
+  /// passed initially will act as an "item 0" for the total set of
+  /// items being reported on.
   ///
   /// @param [in] total The total units of work to be done if specified, if
   /// set to std::nullopt then an indeterminate progress indicator should be