Correct nxdl2rst.py: restore indentation recognition to preserve ReSTful layout.
Except for removing a fixed-length indentation determined from the first
line, <doc> blocks are copied literally into the output .rst files.
This implies that the tag "<doc>" must be followed by newline plus indentation,
except if the text block is an one-liner. Consequently, many .nxdl.xml files
have been hand-edited to ensure conformant <doc> blocks.