doc: Fix the layout of POSIX-related tables

Several tables in the section on POSIX support have a column
"Supported" which is supposed to indicate whether a feature,
a function, or similar, is supported in Zephyr or not. This
indication is done by placing character "+" in relevant cells.

However, in restructuredText, "+" is a sign for an element of
a bulleted list (as is the case with "*" and several other
characters). This, in turn, causes very inefficient rendering of
all these cells with "+", since restructuredText adds spacing
before and after any bulleted list.

Fix this by replacing all "+" characters by a simple "yes".

Signed-off-by: Aleksandar Markovic <aleksandar.markovic.sa@gmail.com>