Added documentation for disabler objects.

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+---
+short-description: Disabling options
+...
+
+# Disabling parts of the build (available since 0.44.0)
+
+The following is a common fragment found in many projects:
+
+```meson
+dep = dependency('foo')
+
+# In some different directory
+
+lib = shared_library('mylib', 'mylib.c',
+  dependencies : dep)
+
+# And ín a third directory
+
+exe = executable('mytest', 'mytest.c',
+  link_with : lib)
+test('mytest', exe)
+```
+
+This works fine but gets a bit inflexible when you want to make this
+part of the build optional. Basically it reduces to adding `if/else`
+statements around all target invocations. Meson provides a simpler way
+of achieving the same with a disabler object.
+
+A disabler object is created with the `disabler` function:
+
+```meson
+d = disabler()
+```
+
+The only thing you can do to a disabler object is to ask if it has
+been found:
+
+```meson
+f = d.found() # returns false
+```
+
+Any other statement that uses a disabler object will immediately
+return a disabler. For example assuming that `d` contains a disabler
+object then
+
+```meson
+d2 = some_func(d) # value of d2 will be disabler
+d3 = true or d2   # value of d3 will be disabler
+if d              # neither branch is evaluated
+```
+
+Thus to disable every target that depends on the dependency given
+above, you can do something like this:
+
+```meson
+if use_foo_feature
+  d = dependency('foo')
+else
+  d = disabler()
+endif
+```
+
+This concentrates the handling of this option in one place and other
+build definition files do not need to be sprinkled with `if`
+statements.
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