nixos/documentation: split options doc build

most modules can be evaluated for their documentation in a very
restricted environment that doesn't include all of nixpkgs. this
evaluation can then be cached and reused for subsequent builds, merging
only documentation that has changed into the cached set. since nixos
ships with a large number of modules of which only a few are used in any
given config this can save evaluation a huge percentage of nixos
options available in any given config.

in tests of this caching, despite having to copy most of nixos/, saves
about 80% of the time needed to build the system manual, or about two
second on the machine used for testing. build time for a full system
config shrank from 9.4s to 7.4s, while turning documentation off
entirely shortened the build to 7.1s.

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/options.nix b/lib/options.nix
index 5d52f065af08..53001a3113f9 100644
--- a/lib/options.nix
+++ b/lib/options.nix
@@ -177,7 +177,7 @@ rec {
         docOption = rec {
           loc = opt.loc;
           name = showOption opt.loc;
-          description = opt.description or (lib.warn "Option `${name}' has no description." "This option has no description.");
+          description = opt.description or null;
           declarations = filter (x: x != unknownModule) opt.declarations;
           internal = opt.internal or false;
           visible =