Fixup description of substituters (#8291)

Introduce what substituters "are" in the configuration option entry.
Remove arbitrary line breaks for easier editing in the future.
Link glossary some more.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: John Ericson <git@JohnEricson.me>

doc/manual/src/glossary.md

@@ -112,9 +112,10 @@
     from some server.
 
   - [substituter]{#gloss-substituter}\
-    A *substituter* is an additional store from which Nix will
+    An additional [store]{#gloss-store} from which Nix can obtain store objects instead of building them.
-    copy store objects it doesn't have.  For details, see the
+    Often the substituter is a [binary cache](#gloss-binary-cache), but any store can serve as substituter.
-    [`substituters` option](./command-ref/conf-file.md#conf-substituters).
+
+    See the [`substituters` configuration option](./command-ref/conf-file.md#conf-substituters) for details.
 
     [substituter]: #gloss-substituter
 

src/libstore/globals.hh

@@ -691,20 +691,19 @@ public:
         Strings{"https://cache.nixos.org/"},
         "substituters",
         R"(
-          A list of [URLs of Nix stores](@docroot@/command-ref/new-cli/nix3-help-stores.md#store-url-format)
+          A list of [URLs of Nix stores](@docroot@/command-ref/new-cli/nix3-help-stores.md#store-url-format) to be used as substituters, separated by whitespace.
-          to be used as substituters, separated by whitespace.
+          A substituter is an additional [store]{@docroot@/glossary.md##gloss-store} from which Nix can obtain [store objects](@docroot@/glossary.md#gloss-store-object) instead of building them.
-          Substituters are tried based on their Priority value, which each substituter can set
-          independently. Lower value means higher priority.
-          The default is `https://cache.nixos.org`, with a Priority of 40.
 
-          At least one of the following conditions must be met for Nix to use
+          Substituters are tried based on their priority value, which each substituter can set independently.
-          a substituter:
+          Lower value means higher priority.
+          The default is `https://cache.nixos.org`, which has a priority of 40.
+
+          At least one of the following conditions must be met for Nix to use a substituter:
 
           - the substituter is in the [`trusted-substituters`](#conf-trusted-substituters) list
           - the user calling Nix is in the [`trusted-users`](#conf-trusted-users) list
 
-          In addition, each store path should be trusted as described
+          In addition, each store path should be trusted as described in [`trusted-public-keys`](#conf-trusted-public-keys)
-          in [`trusted-public-keys`](#conf-trusted-public-keys)
         )",
         {"binary-caches"}};