This comes up now and again when people try do do something like:
meson.build:
```meson
my_sources = ['foo.c']
subdir('subdir')
executable('foo', my_sources)
```
subdir/meson.build:
```meson
my_sources += ['bar.c']
```
The "if" statement only covers a small set of the possible ways in
which conditionals can be written, since it leaves the use of
"and", "or" and "not" to the "Logical Operations" section. However,
this is likely to be of interest to those reading about "if" statments,
so move the "logical operations" section up to immediately follow it.
This change also puts in the use of the "!=" operator in the example
to widen the variety of combinations shown.
In CommonMark, there are no backslash escapes in code spans, so only two backslashes in the source document are necessary to produce two backslashes in the output document.
This allows us to more easily have the documentation in sync with
the source code as people will have to document new features etc
right at the time where they implement it.