llext: llext_load: document memory management policy

The do_llext_load function is responsible for loading an extension from a
file, and for this purpose it calls a number of functions that a) allocate
memory, and b) can fail. This creates the opportunity for memory leaks if
the error paths are not handled correctly.

This commit adds a comment at the beginning of the function to document
the memory management policy that has to be followed in this file:
cleanup is not performed in the error paths, and all memory is freed at
the end of the do_llext_load() function, both in the case of error and of
successful loading.

As an improvement, the symbol table is not freed if the LLEXT log level
is set to debug, so that it can be used, for example, to inspect the
symbols of the loaded extension.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>