docs: why 'ninja test' depends on build_by_default targets [skip ci]
Fixes
@@ -320,8 +320,12 @@ is the name of this target and the keyword arguments are the
following.
- `build_by_default` *(added 0.38)* causes, when set to true, to
- have this target be built by default, that is, when invoking plain
- `ninja`; the default value is false
+ have this target be built by default. This means it will be built when
+ `ninja` is called without any arguments or asked to build a target
+ like `ninja test` that depends on ninja's [default
+ target](https://ninja-build.org/manual.html#_default_target_statements)
+ set to `all` by meson. The same behavior applies for backends other
+ than `ninja`. The default value is `false`.
*(changed in 0.50)* if `build_by_default` is explicitly set to false, `install`
will no longer override it. If `build_by_default` is not set, `install` will
still determine its default.
