docs: fix reference to #noibss in v2018.2.1 release notes

Uncovered by sphinx-build:
docs/releases/v2018.2.1.rst:19: WARNING: Too many autonumbered footnote references: only 0 corresponding footnotes available.
docs/releases/v2018.2.1.rst:19: WARNING: Unknown target name: "noibss".
docs/releases/v2018.2.1.rst:26: WARNING: Unknown target name: "noibss".
docs/releases/v2018.2.1.rst:30: WARNING: Unknown target name: "noibss".
docs/releases/v2018.2.1.rst:34: WARNING: Unknown target name: "noibss".
docs/releases/v2018.2.1.rst:35: WARNING: Unknown target name: "noibss".

docs/releases/v2018.2.1.rst

@@ -34,6 +34,10 @@ ramips-mt76x8
   - Archer C50 (v3, v4) [#noibss]_
   - TL-WR841N (v13) [#noibss]_
 
+.. [#noibss]
+  Device or target does not support AP+IBSS mode: This device or target will not be built
+  when *GLUON_WLAN_MESH* is set to ``ibss``.
+
 Bugfixes
 ~~~~~~~~