State the no-tool pressure advance contract per firmware

set_filament_pressure_advance's declaration said -1 omits the tool qualifier,
without qualification. That holds on Klipper, Marlin and BBL but not on
RepRapFirmware, where no-tool keeps the historical `D0`: a bare M572 applies to
whichever tool is selected and errors when none is, so omitting the qualifier
would make pressure advance depend on tool-selection state for every RRF user,
none of whom are using IMEX. The behavior is deliberate and unchanged; only the
declaration overstated it. GCodeWriter::set_pressure_advance and the index-space
notes in IMEXHelpers.hpp already described it correctly.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/libslic3r/GCode.hpp

@@ -291,8 +291,10 @@ public:
     std::string     unretract(float extra_retract = 0.f) { return m_writer.unlift() + m_writer.unretract(extra_retract); }
     std::string     set_extruder(unsigned int extruder_id, double print_z, bool by_object=false, int toolchange_temp_override = -1, bool defer_temp_wait = false);
     // Sets the pressure advance of the filament's extruder variant, if enabled for it.
-    // tool addresses one IMEX carriage explicitly; -1 omits the tool qualifier, which is
-    // what every non-IMEX caller wants and what imex_pem_tool_for() returns off IMEX.
+    // tool addresses one IMEX carriage explicitly. -1 means no carriage: it omits the tool
+    // qualifier on Klipper, Marlin and BBL, and keeps RepRapFirmware's historical `D0`, since a
+    // bare M572 there applies to whatever tool is selected and errors when none is. That is
+    // what every non-IMEX caller wants, and what imex_pem_tool_for() returns off IMEX.
     std::string     set_filament_pressure_advance(unsigned int filament_id, int tool = -1);
     bool is_BBL_Printer();
     WipeTowerType wipe_tower_type();