docs/devel/decodetree: Fix formatting in "field examples" table

The rST syntax for a table uses ASCII art to draw the cell boundaries; then inside each cell the text is treated as a body element, so it is rendered the same way as text at the top level of a document. The "field examples" table was assuming a "literal document" format for its cell bodies; this meant that the single line cells were being rendered in plain text, not a fixed width font, and the multi line cells were rendered as definition-lists because of their "second and subsequent lines are indented" layout. Fix this by consistently using inline-code markup for the left column and literal blocks for the right column. (We want to be consistent within each column because a literal block renders differently to inline-code, with a green background.) Reviewed-by: Alex Bennée <alex.bennee@linaro.org> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>