doc: fix over/underfull hboxes in PDF

doc: fix over/underfull hboxes in PDF

This adds some cosmetic changes to make the PDF User Manual look
proper.

Building it now requires Texinfo 5.0 which shouldn't be problematic
according to [0]. Commit 79fdeb37f486f74658f1eaf658abac8efb3eba6a is
effectively reverted.

[0] https://repology.org/project/texinfo/versions

Change-Id: I990bc23bdb53d24c302b26d74fd770ea738e4096
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/5995
Reviewed-by: Jonathan McDowell <noodles-openocd@earth.li>
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>