Add a potential cause raising ParseIntError.

Initially, I wanted to add it directly to the documentation of `str. parse()' method, I finally found that it was more relevant (I hope so?) to directly document the structure in question. I've added a scenario, in which we could all get caught at least once, to make it easier to diagnose the problem when parsing integers.

src/libcore/num/mod.rs

@@ -3846,6 +3846,12 @@ fn from_str_radix<T: FromStrRadixHelper>(src: &str, radix: u32) -> Result<T, Par
 /// This error is used as the error type for the `from_str_radix()` functions
 /// on the primitive integer types, such as [`i8::from_str_radix`].
 /// 
+/// # Potential causes
+///
+/// Among other causes, `ParseIntError` can be thrown because of leading or trailing whitespaces in the string e.g. when it is getted from the standard input. 
+/// Using the [`str.trim()`] method ensures that no whitespace remains before parsing.
+///
+/// [`str.trim()`]: ../../std/primitive.str.html#method.trim
 /// [`i8::from_str_radix`]: ../../std/primitive.i8.html#method.from_str_radix
 #[derive(Debug, Clone, PartialEq, Eq)]
 #[stable(feature = "rust1", since = "1.0.0")]