chore: fix a typo in the docstring for SeqRight and add documentation to getLevel (#10866)

src/Init/Prelude.lean

@@ -3703,7 +3703,7 @@ When thinking about `f` as potential side effects, `*>` evaluates first the left
 argument for their side effects, discarding the value of the left argument and returning the value
 of the right argument.
 
-For most applications, `Applicative` or `Monad` should be used rather than `SeqLeft` itself.
+For most applications, `Applicative` or `Monad` should be used rather than `SeqRight` itself.
 -/
 class SeqRight (f : Type u → Type v) : Type (max (u+1) v) where
   /--

src/Lean/Meta/InferType.lean

@@ -129,6 +129,12 @@ private def inferProjType (structName : Name) (idx : Nat) (e : Expr) : MetaM Exp
 def throwTypeExpected {α} (type : Expr) : MetaM α :=
   throwError "type expected{indentExpr type}"
 
+/--
+If `type : sort` and `sort` reduces to `Sort u` for some `u`, then `getLevel type` returns `u`.
+
+If `sort` is an assignable MVar, then `getLevel type` produces a fresh level metavariable `?u`,
+assigns the MVar to `Sort ?u` and returns `?u`.
+-/
 def getLevel (type : Expr) : MetaM Level := do
   let typeType ← inferType type
   let typeType ← whnfD typeType