Support free functions in disallowed-methods lint

In other words, support:

`disallowed_methods = ["alloc::vec::Vec::new"]` (a free function) in
addition to
`disallowed_methods = ["alloc::vec::Vec::leak"]` (a method).

Improve the documentation to clarify that users must specify the full
qualified path for each disallowed function, which can be confusing for
reexports. Include an example `clippy.toml`.

Simplify the actual lint pass so we can reuse `utils::fn_def_id`.

tests/ui-toml/toml_disallowed_method/clippy.toml

@@ -1 +1 @@
-disallowed-methods = ["core::iter::traits::iterator::Iterator::sum", "regex::re_unicode::Regex::is_match"]
+disallowed-methods = ["core::iter::traits::iterator::Iterator::sum", "regex::re_unicode::Regex::is_match", "regex::re_unicode::Regex::new"]

tests/ui-toml/toml_disallowed_method/conf_disallowed_method.rs

@@ -4,10 +4,9 @@ extern crate regex;
 use regex::Regex;
 
 fn main() {
-    let a = vec![1, 2, 3, 4];
     let re = Regex::new(r"ab.*c").unwrap();
-
     re.is_match("abc");
 
+    let a = vec![1, 2, 3, 4];
     a.iter().sum::<i32>();
 }

tests/ui-toml/toml_disallowed_method/conf_disallowed_method.stderr

@@ -1,16 +1,22 @@
-error: use of a disallowed method `regex::re_unicode::Regex::is_match`
-  --> $DIR/conf_disallowed_method.rs:10:5
+error: use of a disallowed method `regex::re_unicode::Regex::new`
+  --> $DIR/conf_disallowed_method.rs:7:14
    |
-LL |     re.is_match("abc");
-   |     ^^^^^^^^^^^^^^^^^^
+LL |     let re = Regex::new(r"ab.*c").unwrap();
+   |              ^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::disallowed-method` implied by `-D warnings`
 
+error: use of a disallowed method `regex::re_unicode::Regex::is_match`
+  --> $DIR/conf_disallowed_method.rs:8:5
+   |
+LL |     re.is_match("abc");
+   |     ^^^^^^^^^^^^^^^^^^
+
 error: use of a disallowed method `core::iter::traits::iterator::Iterator::sum`
-  --> $DIR/conf_disallowed_method.rs:12:5
+  --> $DIR/conf_disallowed_method.rs:11:5
    |
 LL |     a.iter().sum::<i32>();
    |     ^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors