Rollup merge of rust-lang#102058 - mqudsi:path_extension_docs, r=thomcc

Clarify Path::extension() semantics in docs abstract

State up-front and center what shape the returned extension will have, without making the user read through the description and examples.

This is a doc-only change. There are no changes to the API contract and the clarification is in line with what was already stated/promised in the existing doc text - just clarified, summarized, and served bright and early.

Rationale: Various frameworks and libraries for different platforms have their different conventions as to whether an "extension" is ".ext" or just "ext" and anyone that's had to deal with this ambiguity in the past is always double- or triple-checking to make sure the function call returns an extension that matches the expected semantics. Offer the answer to this important question right off the bat instead of making them dig to find it.

```@rustbot``` label +A-docs

Author: Dylan-DPC

library/std/src/path.rs

@@ -2401,7 +2401,7 @@ impl Path {
         self.file_name().map(split_file_at_dot).and_then(|(before, _after)| Some(before))
     }
 
-    /// Extracts the extension of [`self.file_name`], if possible.
+    /// Extracts the extension (without the leading dot) of [`self.file_name`], if possible.
     ///
     /// The extension is:
     ///