Adjust visibility of codependent MutableRepo and CommitBuilder functions

MutableRepo and CommitBuilder both define public (now crate-public) functions
which should only be called by each other. This commit adds documentation and
restricts visibility of these functions to the jj_lib crate. It might be even
better to move CommitBuilder to the same module as MutableRepo so that these
codependent functions can be private to the module to avoid misuse.

lib/src/commit_builder.rs

@@ -32,7 +32,7 @@ pub struct CommitBuilder<'repo> {
 }
 
 impl CommitBuilder<'_> {
-    // Use MutRepo::new_commit() instead
+    /// Only called from [`MutRepo::new_commit`]. Use that function instead.
     pub(crate) fn for_new_commit<'repo>(
         mut_repo: &'repo mut MutableRepo,
         settings: &UserSettings,
@@ -62,7 +62,7 @@ impl CommitBuilder<'_> {
         }
     }
 
-    // Use MutRepo::rewrite_commit() instead
+    /// Only called from [`MutRepo::rewrite_commit`]. Use that function instead.
     pub(crate) fn for_rewrite_from<'repo>(
         mut_repo: &'repo mut MutableRepo,
         settings: &UserSettings,

lib/src/repo.rs

@@ -771,11 +771,12 @@ impl MutableRepo {
             && self.view() == &self.base_repo.view)
     }
 
-    pub fn consume(self) -> (Box<dyn MutableIndex>, View) {
+    pub(crate) fn consume(self) -> (Box<dyn MutableIndex>, View) {
         self.view.ensure_clean(|v| self.enforce_view_invariants(v));
         (self.index, self.view.into_inner())
     }
 
+    /// Returns a [`CommitBuilder`] to write new commit to the repo.
     pub fn new_commit(
         &mut self,
         settings: &UserSettings,
@@ -785,6 +786,7 @@ impl MutableRepo {
         CommitBuilder::for_new_commit(self, settings, parents, tree_id)
     }
 
+    /// Returns a [`CommitBuilder`] to rewrite an existing commit in the repo.
     pub fn rewrite_commit(
         &mut self,
         settings: &UserSettings,
@@ -795,7 +797,8 @@ impl MutableRepo {
         // `self.rewritten_commits`
     }
 
-    pub fn write_commit(
+    /// Only called from [`CommitBuilder::write`]. Use that function instead.
+    pub(crate) fn write_commit(
         &mut self,
         commit: backend::Commit,
         sign_with: Option<&mut SigningFn>,