Fix doc links pointing to private items (ZcashFoundation#6944)

Our documentation for some public items contains links to private items.
These links resolve fine because we use the `--document-private-flags`,
but still generate warnings during the compilation of the docs.

This commit (hopefully) correctly suppresses the warnings, and updates
CI.

.cargo/config.toml

@@ -55,12 +55,6 @@ rustflags = [
     # Documentation
     "-Wmissing_docs",
 
-    # These rustdoc -A and -W settings must be the same as the RUSTDOCFLAGS in:
-    # https://github.com/ZcashFoundation/zebra/blob/main/.github/workflows/lint.yml#L152
-
-    # Links in public docs can point to private items.
-    "-Arustdoc::private_intra_doc_links",
-
     # TODOs:
     # `cargo fix` might help do these fixes,
     # or add a config.toml to sub-directories which should allow these lints,
@@ -82,3 +76,12 @@ rustflags = [
     # fix hidden lifetime parameters
     #"-Wrust_2018_idioms",
 ]
+
+[build]
+rustdocflags = [
+    # The -A and -W settings must be the same as the `RUSTDOCFLAGS` in:
+    # https://github.com/ZcashFoundation/zebra/blob/main/.github/workflows/lint.yml#L151
+
+    # Links in public docs can point to private items.
+    "-Arustdoc::private_intra_doc_links",
+]

.github/workflows/lint.yml

@@ -145,8 +145,8 @@ jobs:
     # cargo doc doesn't support '--  -D warnings', so we have to add it here
     # https://github.com/rust-lang/cargo/issues/8424#issuecomment-774662296
     #
-    # These -A and -W settings must be the same as the rustdoc settings in:
-    # https://github.com/ZcashFoundation/zebra/blob/main/.cargo/config.toml#L53
+    # The -A and -W settings must be the same as the `rustdocflags` in:
+    # https://github.com/ZcashFoundation/zebra/blob/main/.cargo/config.toml#L87
     env:
       RUSTDOCFLAGS: -D warnings -A rustdoc::private_intra_doc_links