nixos/manpages: Explain -I option and how to build manpages

When running nixos-rebuild -I, the man page just said "-I path" which could be interpreted as
 "just a path to nixpkgs", which in fact it actually has the same meaning as NIX_PATH.

This is now made clear in the manual, so that when grepping "-I" and "NIX_PATH" one quickly finds
 the format of the option.

I don't know how to link to the "nix manual" as stated in the docbook, so I left that as it is.

Additionally, it wasn't clear to me how to actually build the man pages and view the changes I made.
That's now in the contributing-to-this-manual.chapter.md.

1 files changed, 23 insertions, 1 deletions

diff --git a/nixos/doc/manual/contributing-to-this-manual.chapter.md b/nixos/doc/manual/contributing-to-this-manual.chapter.md
index 26813d1042d69..557599809222a 100644
--- a/nixos/doc/manual/contributing-to-this-manual.chapter.md
+++ b/nixos/doc/manual/contributing-to-this-manual.chapter.md
@@ -1,6 +1,6 @@
 # Contributing to this manual {#chap-contributing}
 
-The DocBook and CommonMark sources of NixOS' manual are in the [nixos/doc/manual](https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual) subdirectory of the [Nixpkgs](https://github.com/NixOS/nixpkgs) repository.
+The [DocBook] and CommonMark sources of the NixOS manual are in the [nixos/doc/manual](https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual) subdirectory of the [Nixpkgs](https://github.com/NixOS/nixpkgs) repository.
 
 You can quickly check your edits with the following:
 
@@ -11,3 +11,25 @@ $ nix-build nixos/release.nix -A manual.x86_64-linux
 ```
 
 If the build succeeds, the manual will be in `./result/share/doc/nixos/index.html`.
+
+**Contributing to the man pages**
+
+The man pages are written in [DocBook] which is XML.
+
+To see what your edits look like:
+
+```ShellSession
+$ cd /path/to/nixpkgs
+$ nix-build nixos/release.nix -A manpages.x86_64-linux
+```
+
+You can then read the man page you edited by running
+
+```ShellSession
+$ man --manpath=result/share/man nixos-rebuild # Replace nixos-rebuild with the command whose manual you edited
+```
+
+If you're on a different architecture that's supported by NixOS (check nixos/release.nix) then replace `x86_64-linux` with the architecture.
+`nix-build` will complain otherwise, but should also tell you which architecture you have + the supported ones.
+
+[DocBook]: https://en.wikipedia.org/wiki/DocBook