docs: rv-virt: document elf, libcxx64 and virtual LEDs

Document the purpose of the rv-virt elf and libcxx64 configurations and clarify that the LEDs used by leds64 are virtual/log-only and are not backed by a NuttX GPIO controller.

This addresses issue #20174.

Documentation/platforms/risc-v/qemu-rv/boards/rv-virt/index.rst

@@ -143,6 +143,21 @@ To run it with QEMU, use the following command::
       -mon chardev=con,mode=readline \
       -bios none -kernel nuttx
 
+elf
+---
+
+This configuration uses ``apps/examples/elf`` in order to test the ELF loader
+on 32-bit RISC-V. It boots directly into the ELF example and does not provide
+the NuttShell (NSH).
+
+libcxx64
+--------
+
+This configuration provides the NuttShell (NSH) with the C++ standard library
+on 64-bit RISC-V. The C++ test at ``apps/testing/cxxtest`` can be run with::
+
+    nsh> cxxtest
+
 lvgl64_vector
 -------------
 
@@ -295,6 +310,10 @@ leds64
 
 Similar to the `nsh64`_ configuration, but with User LEDs support for 64-bit RISC-V.
 
+The LEDs are implemented as virtual LEDs and their state is reported through
+the system log. The current ``rv-virt`` implementation does not provide
+a NuttX GPIO controller for these LEDs.
+
 leds64_rust
 -----------