Fix doc nits

Many tiny changes to stdlib doc comments to make them consistent (for example
"Returns foo", rather than "Return foo", per RFC1574), adding missing periods, paragraph
breaks, backticks for monospace style, and other minor nits.

https://github.com/rust-lang/rfcs/blob/master/text/1574-more-api-documentation-conventions.md#appendix-a-full-conventions-text

library/std/src/process.rs

@@ -2056,7 +2056,7 @@ impl ExitCode {
     // representation of an ExitCode
     //
     // More info: https://internals.rust-lang.org/t/mini-pre-rfc-redesigning-process-exitstatus/5426
-    /// Convert an `ExitCode` into an i32
+    /// Converts an `ExitCode` into an i32
     #[unstable(
         feature = "process_exitcode_internals",
         reason = "exposed only for libstd",
@@ -2079,7 +2079,7 @@ impl Default for ExitCode {
 
 #[stable(feature = "process_exitcode", since = "1.61.0")]
 impl From<u8> for ExitCode {
-    /// Construct an `ExitCode` from an arbitrary u8 value.
+    /// Constructs an `ExitCode` from an arbitrary u8 value.
     fn from(code: u8) -> Self {
         ExitCode(imp::ExitCode::from(code))
     }