Export the "specific conditions" requirement and reword paragraph.

Mostly some Bikeshed cleanup as a follow-up to #434. That pull request
removed the `<dfn>` for "mandatory conditions", which continued to be
referenced by the "specific conditions" item in the "can expose sensor
readings" list.

Remove the reference to it and, at the same time, export the `<dfn>` for
"specific conditions" so that it can actually be used by extension
specifications.

Author: Raphael Kubo da Costa

index.bs

@@ -755,8 +755,8 @@ all of the following are true:
  - |document|'s [=visibility state=] is "visible".
  - The [=currently focused area=] belongs to a document whose origin is [=same
    origin-domain=] with |document|'s origin.
- - <dfn>Specific conditions</dfn>: The [=extension specifications=] that add new
-   [=mandatory conditions|conditions=] hook into this specification at this point.
+ - <dfn export>Specific conditions</dfn>: [=Extension specifications=] may add new
+   conditions to this list to have stricter requirements for their sensor types.
 
 Note: In addition to the conditions above, it is important to note that {{Sensor}}
 subclasses invoke the [=check sensor policy-controlled features=] operation in their