rp2/modrp2: Disable other core, shorten delay to 8us in bootsel_button.

This function seems to work fine in multi-core applications now.

The delay is now in units of microseconds instead of depending on the clock
speed, and is adjustable by board configuration headers.

Also added documentation.

docs/library/rp2.rst

@@ -66,6 +66,17 @@ For running PIO programs, see :class:`rp2.StateMachine`.
     >>> rp2.asm_pio_encode("set(0, 1)", 0)
     57345
 
+.. function:: bootsel_button()
+
+    Temporarily turns the QSPI_SS pin into an input and reads its value,
+    returning 1 for low and 0 for high.
+    On a typical RP2040 board with a BOOTSEL button, a return value of 1
+    indicates that the button is pressed.
+
+    Since this function temporarily disables access to the external flash
+    memory, it also temporarily disables interrupts and the other core to
+    prevent them from trying to execute code from flash.
+
 .. class:: PIOASMError
 
     This exception is raised from `asm_pio()` or `asm_pio_encode()` if there is