Manual improvements

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

doc/manual/src/command-ref/env-common.md

@@ -7,7 +7,7 @@ Most Nix commands interpret the following environment variables:
     `nix-shell`. It can have the values `pure` or `impure`.
 
   - [`NIX_PATH`]{#env-NIX_PATH}\
-    A colon-separated list of directories used to look up Nix
+    A colon-separated list of directories used to look up the location of Nix
     expressions enclosed in angle brackets (i.e., `<path>`),
      e.g. `/home/eelco/Dev:/etc/nixos`. It can be extended using the
     `-I` option. For more information about the semantics of the Nix

src/libcmd/common-eval-args.cc

@@ -34,8 +34,8 @@ MixEvalArgs::MixEvalArgs()
         .shortName = 'I',
         .description = R"(
   Add *path* to the Nix search path. The Nix search path is
-  initialized from the colon-separated `NIX_PATH` environment
-  variable, and is used to look up Nix expressions enclosed in angle
+  initialized from the colon-separated [`NIX_PATH`](./env-common.md#env-NIX_PATH) environment
+  variable, and is used to look up the location of Nix expressions using [paths](../language/values.md#type-path) enclosed in angle
   brackets (i.e., `<nixpkgs>`). For instance, if the Nix search path
   consists of the entries