doc: fix anchor links in and to glossary

doc/manual/src/glossary.md

@@ -156,6 +156,8 @@
     to path `Q`, then `Q` is in the closure of `P`. Further, if `Q`
     references `R` then `R` is also in the closure of `P`.
 
+    [closure]: #gloss-closure
+
   - [output path]{#gloss-output-path}\
     A [store path] produced by a [derivation].
 
@@ -172,6 +174,8 @@
     - The store path is listed in the Nix database as being valid.
     - All paths in the store path's [closure] are valid.
 
+    [validity]: #gloss-validity
+
   - [user environment]{#gloss-user-env}\
     An automatically generated store object that consists of a set of
     symlinks to “active” applications, i.e., other store paths. These