This patch updates the "Source Organisation" section of the
Programmers Guide to add an note that the code in the
drivers/net/pmd/base directories should not be modifed
by the user apart from the pmd_osdep.[ch] files.
Also fixes some grammar and rst issues in that section.
Signed-off-by: John McNamara <john.mcnamara@intel.com>
0e03fe7459