Improve snapshot documentation

Author: mpilgrem

doc/global_flags.md

@@ -227,8 +227,12 @@ At the command line (only):
 *   `--snapshot lts-<major_version>` specifies the latest Stackage LTS Haskell
     snapshot with the specified major version;
 *   `--snapshot lts` specifies, from those with the greatest major version, the
-    latest Stackage LTS Haskell snapshot; and
-*   `--snapshot nightly` specifies the most recent Stackage Nightly snapshot.
+    latest Stackage LTS Haskell snapshot;
+*   `--snapshot nightly` specifies the most recent Stackage Nightly snapshot;
+    and
+*   `--snapshot global` specifies the snapshot specified by the project-level
+    configuration file in the `global-project` directory in the
+    [Stack root](stack_root.md#global-project-directory).
 
 ## `--stack-colors` or `--stack-colours` options
 

doc/yaml_configuration.md

@@ -116,9 +116,6 @@ Each of these snapshots will also determine what constraints are placed on the
 compiler version. See the [compiler-check](#compiler-check) option for some
 additional control over compiler version.
 
-The `snapshot` key specifies a snapshot location. For further information, see
-the [snapshot and package location](pantry.md) documentation.
-
 ### resolver
 
 `resolver` and [`snapshot`](#snapshot) are synonyms. Only one of these keys is