repository_rules.md: update documentation on return values

Since commit 97132c0925031e1b1fb8e9e5dde2233daef9d97b, repository rule
implementations are allowed to return other values than `None`. Describe
the currently supported values and their intended meaning.

Change-Id: I63dbbff4b232b36861aa2a6cfdc69352fa971f4d
PiperOrigin-RevId: 200707564

1 files changed, 9 insertions, 2 deletions

diff --git a/site/docs/skylark/repository_rules.md b/site/docs/skylark/repository_rules.md
index 8e492ac091..44e40e3240 100644
--- a/site/docs/skylark/repository_rules.md
+++ b/site/docs/skylark/repository_rules.md
@@ -51,8 +51,15 @@ If an attribute name starts with `_` it is private and users cannot set it.
 
 Every repository rule requires an `implementation` function. It contains the
 actual logic of the rule and is executed strictly in the Loading Phase.
-The function has exactly one input parameter, `repository_ctx`, and should
-always returns `None`. The input parameter `repository_ctx` can be used to
+
+The function has exactly one input parameter, `repository_ctx`. The function
+returns either `None` to signify that the rule is reproducible, or a dict with a
+set of parameters for that rule that would turn that rule into a reproducible
+one generating the same repository. For example, for a rule tracking a git
+repository that would mean returning a specific commit identifier instead of a
+floating branch that was originally specified.
+
+The input parameter `repository_ctx` can be used to
 access attribute values, and non-hermetic functions (finding a binary,
 executing a binary, creating a file in the repository or downloading a file
 from the Internet). See [the library](lib/repository_ctx.html) for more