doc: update help message

Update the help message used for 'make help':

  Documentation targets:
    Linux kernel internal documentation in different formats from ReST:
=>
    U-Boot documentation in different formats from ReST:

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
1 file changed