Add documentation for T? command

_gcode/T-mmu2.md

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+---
+tag: t0001
+title: MMU2 Special Commands
+brief: MMU2 special filament loading commands
+author: thinkyhead
+since: 2.0.0
+
+group: control
+
+codes: [ 'T?', Tc, Tx ]
+
+examples:
+  - pre: Select a new filament, set temperature, wait for heating, load to the nozzle
+    code:
+    - T3
+    - M104 S210
+    - T?
+  - pre: Select a new filament, load it up to the extruder gears, wait for heating, then load to the nozzle
+    code:
+    - T2
+    - Tx
+    - M104 S208
+    - Tc
+
+---
+For the MMU2 and clones the `T0`-`T7` commands select a new filament. After this command you must extrude at least 38.1 mm of filament at feedrate 19.02 mm/s to reach the extruder. (The distance to the extruder may differ for your particular machine.)
+
+#### Special Commands
+
+- `T?` : (Requires `MMU2_MENUS`) Wait for target temperature, then load the filament all the way to the nozzle. G-code to extrude more is not needed.
+- `Tx` : (Requires `MMU2_MENUS`) Load the filament up to the direct-drive extruder gears. No heating is required.
+- `Tc` : Wait for target temperature, then load the filament from the extruder gears up to the nozzle. G-code to extrude more is not needed.

_gcode/T.md

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+---
+tag: t0000
+title: Select or Report Tool
+brief: Set or report the current extruder or other tool
+author: shitcreek
+contrib: ansonl, thinkyhead
+
+group: control
+
+codes: [ T0, T1, T2, T3, T4, T5, T6, T7 ]
+
+parameters:
+  - tag: F
+    optional: true
+    description: Movement feedrate for the tool-change.
+    values:
+      - tag: feedrate
+        type: float
+  - tag: S
+    optional: true
+    description: "Specify movement in XY after the tool-change. (Default: allow)"
+    values:
+      - tag: 0
+        description: Allow movement in XY
+        type: bool
+      - tag: 1
+        description: Prevent movement in XY
+        type: bool
+
+examples:
+  - pre: Switch to tool 1 with XY move feedrate of 30mm/s
+    code: T1 F1800
+  - pre: Switch to tool 3, disallowing XY moves
+    code: T3 S1
+  - pre: Report the current tool
+    code: T
+---
+Use `T0`, `T1`, etc. to switch to the respective physical or virtual tool. See [Universal Tool Change Settings](/docs/configuration/configuration.html#universal-tool-change-settings) in `Configuration_adv.h` for more details.
+
+In Marlin 2.1.3 and up you can use `T` with no tool number to report the current tool index to the serial console.
+
+#### For MMU 2 (and Clones)
+See the [MMU2 Special Commands](T-mmu2.html) page for information about special commands `T?`, `Tc`, and `Tx`.
+
+For the MMU2 and clones the `T0`-`T7` commands select a new filament. After this command you must extrude at least 38.1 mm of filament at feedrate 19.02 mm/s to reach the extruder. (The distance to the extruder may differ for your particular machine.)