Rollup merge of rust-lang#46240 - SimonSapin:from_str_radix-docs, r=estebank

Expand docs of <$Int>::from_str_radix, based on that of char::to_digit

Author: kennytm

src/libcore/num/mod.rs

@@ -132,7 +132,14 @@ macro_rules! int_impl {
 
         /// Converts a string slice in a given base to an integer.
         ///
+        /// The string is expected to be an optional `+` or `-` sign
+        /// followed by digits.
         /// Leading and trailing whitespace represent an error.
+        /// Digits are a subset of these characters, depending on `radix`:
+        ///
+        /// * `0-9`
+        /// * `a-z`
+        /// * `A-Z`
         ///
         /// # Panics
         ///
@@ -1301,7 +1308,18 @@ macro_rules! uint_impl {
 
         /// Converts a string slice in a given base to an integer.
         ///
+        /// The string is expected to be an optional `+` sign
+        /// followed by digits.
         /// Leading and trailing whitespace represent an error.
+        /// Digits are a subset of these characters, depending on `radix`:
+        ///
+        /// * `0-9`
+        /// * `a-z`
+        /// * `A-Z`
+        ///
+        /// # Panics
+        ///
+        /// This function panics if `radix` is not in the range from 2 to 36.
         ///
         /// # Examples
         ///