some corrections to the building the manual documentation

Building the LAMMPS manual

**************************

Depending on how you obtained LAMMPS, the doc directory has 2 or 3

sub-directories and optionally 2 PDF files and 2 e-book format files:

Depending on how you obtained LAMMPS, the doc directory has up

to 6 sub-directories, 2 Nroff files, and optionally 2 PDF files

plus 2 e-book format files:

.. parsed-literal::

   src             # content files for LAMMPS documentation

   html            # HTML version of the LAMMPS manual (see html/Manual.html)

   tools           # tools and settings for building the documentation

   utils           # tools and settings for building the documentation

   docenv          # virtualenv for processing the manual sources

   doctrees        # temporary data from processing the manual

   mathjax         # code and fonts for rendering math in html

   Manual.pdf      # large PDF version of entire manual

   Developer.pdf   # small PDF with info about how LAMMPS is structured

   LAMMPS.epub     # Manual in ePUB e-book format

   LAMMPS.mobi     # Manual in MOBI e-book format

   lammps.1        # man page for the lammps command

   msi2lmp.1       # man page for the msi2lmp command

If you downloaded LAMMPS as a tarball from the web site, all these

directories and files should be included.

If you downloaded LAMMPS as a tarball from the web site, the html folder

and the PDF files should be included.

If you downloaded LAMMPS from the public git repository, then the HTML

and PDF files are not included.  Instead you need to create them, in one