feat!: Allow resetting abouts/help to default

I dislike the inconsistency with only a few fields providing this (this
and `help_heading`) but this is to address a specific bug.  We need to
visit this, along with iterators (clap-rs#2870) and string handling (clap-rs#2150).

`Arg` came along for the ride because the derive logic is applied to
both.  `PossibleValue` didn't need it because we filter out `long_help`.

BREAKING CHANGE: We changed the signatures for `App::about`,
`App::long_about`, `Arg::help`, and `Arg::long_help` from accepting
anything `Into<&str>` to `&str`.

src/build/app/mod.rs

@@ -896,8 +896,8 @@ impl<'help> App<'help> {
     ///     .about("Does really amazing things for great people")
     /// # ;
     /// ```
-    pub fn about<S: Into<&'help str>>(mut self, about: S) -> Self {
-        self.about = Some(about.into());
+    pub fn about<O: Into<Option<&'help str>>>(mut self, about: O) -> Self {
+        self.about = about.into();
         self
     }
 
@@ -920,8 +920,8 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     /// [`App::about`]: App::about()
-    pub fn long_about<S: Into<&'help str>>(mut self, about: S) -> Self {
-        self.long_about = Some(about.into());
+    pub fn long_about<O: Into<Option<&'help str>>>(mut self, long_about: O) -> Self {
+        self.long_about = long_about.into();
         self
     }
 

src/build/arg/mod.rs

@@ -2712,8 +2712,8 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::long_help`]: Arg::long_help()
     #[inline]
-    pub fn help(mut self, h: &'help str) -> Self {
-        self.help = Some(h);
+    pub fn help(mut self, h: impl Into<Option<&'help str>>) -> Self {
+        self.help = h.into();
         self
     }
 
@@ -2773,8 +2773,8 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::help`]: Arg::help()
     #[inline]
-    pub fn long_help(mut self, h: &'help str) -> Self {
-        self.long_help = Some(h);
+    pub fn long_help(mut self, h: impl Into<Option<&'help str>>) -> Self {
+        self.long_help = h.into();
         self
     }