doc: minimize mentions of nix-env -i without -A in nixpkgs manual

doc/languages-frameworks/emscripten.section.md

@@ -15,12 +15,12 @@ Modes of use of `emscripten`:
 
    If you want to work with `emcc`, `emconfigure` and `emmake` as you are used to from Ubuntu and similar distributions you can use these commands:
 
-    * `nix-env -i emscripten`
+    * `nix-env -f "<nixpkgs>" -iA emscripten`
     * `nix-shell -p emscripten`
 
 * **Declarative usage**:
 
-    This mode is far more power full since this makes use of `nix` for dependency management of emscripten libraries and targets by using the `mkDerivation` which is implemented by `pkgs.emscriptenStdenv` and `pkgs.buildEmscriptenPackage`. The source for the packages is in `pkgs/top-level/emscripten-packages.nix` and the abstraction behind it in `pkgs/development/em-modules/generic/default.nix`.
+    This mode is far more power full since this makes use of `nix` for dependency management of emscripten libraries and targets by using the `mkDerivation` which is implemented by `pkgs.emscriptenStdenv` and `pkgs.buildEmscriptenPackage`. The source for the packages is in `pkgs/top-level/emscripten-packages.nix` and the abstraction behind it in `pkgs/development/em-modules/generic/default.nix`. From the root of the nixpkgs repository:
     * build and install all packages:
         * `nix-env -iA emscriptenPackages`
 

doc/languages-frameworks/idris.section.md

@@ -5,10 +5,7 @@
 The easiest way to get a working idris version is to install the `idris` attribute:
 
 ```ShellSession
-$ # On NixOS
-$ nix-env -i nixos.idris
-$ # On non-NixOS
-$ nix-env -i nixpkgs.idris
+$ nix-env -f "<nixpkgs>" -iA idris
 ```
 
 This however only provides the `prelude` and `base` libraries. To install idris with additional libraries, you can use the `idrisPackages.with-packages` function, e.g. in an overlay in `~/.config/nixpkgs/overlays/my-idris.nix`:

doc/languages-frameworks/octave.section.md

@@ -24,18 +24,10 @@ You can test building an Octave package as follows:
 $ nix-build -A octavePackages.symbolic
 ```
 
-When building Octave packages with `nix-build`, the `buildOctavePackage` function adds `octave-octaveVersion` to; the start of the package's name attribute.
-
-This can be required when installing the package using `nix-env`:
+To install it into your user profile, run this command from the root of the repository:
 
 ```ShellSession
-$ nix-env -i octave-6.2.0-symbolic
-```
-
-Although, you can also install it using the attribute name:
-
-```ShellSession
-$ nix-env -i -A octavePackages.symbolic
+$ nix-env -f. -iA octavePackages.symbolic
 ```
 
 You can build Octave with packages by using the `withPackages` passed-through function.

doc/languages-frameworks/perl.section.md

@@ -58,13 +58,7 @@ in `all-packages.nix`. You can test building a Perl package as follows:
 $ nix-build -A perlPackages.ClassC3
 ```
 
-`buildPerlPackage` adds `perl-` to the start of the name attribute, so the package above is actually called `perl-Class-C3-0.21`. So to install it, you can say:
-
-```ShellSession
-$ nix-env -i perl-Class-C3
-```
-
-(Of course you can also install using the attribute name: `nix-env -i -A perlPackages.ClassC3`.)
+To install it with `nix-env` instead: `nix-env -f. -iA perlPackages.ClassC3`.
 
 So what does `buildPerlPackage` do? It does the following:
 
@@ -135,9 +129,11 @@ This will remove the `-I` flags from the shebang line, rewrite them in the `use
 Nix expressions for Perl packages can be generated (almost) automatically from CPAN. This is done by the program `nix-generate-from-cpan`, which can be installed as follows:
 
 ```ShellSession
-$ nix-env -i nix-generate-from-cpan
+$ nix-env -f "<nixpkgs>" -iA nix-generate-from-cpan
 ```
 
+Substitute `<nixpkgs>` by the path of a nixpkgs clone to use the latest version.
+
 This program takes a Perl module name, looks it up on CPAN, fetches and unpacks the corresponding package, and prints a Nix expression on standard output. For example:
 
 ```ShellSession