Try to open man page as help for subprocess commands

This makes for example 'sh?' equivalent to 'man sh', unless there actually
exists a 'sh?' executable.

Resolves #3204.

news/subprocess-help.rst

@@ -0,0 +1,23 @@
+**Added:**
+
+* Open man page when requesting help for subprocess commands, e.g. using ``sh?``
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>

xonsh/built_ins.py

@@ -563,6 +563,11 @@ class SubprocSpec:
             raise XonshError(e.format(self.cmd[0]))
         except FileNotFoundError:
             cmd0 = self.cmd[0]
+            if len(self.cmd) == 1 and cmd0.endswith("?"):
+                with contextlib.suppress(OSError):
+                    return self.cls(
+                        ["man", cmd0.rstrip("?")], bufsize=bufsize, **kwargs
+                    )
             e = "xonsh: subprocess mode: command not found: {0}".format(cmd0)
             env = builtins.__xonsh__.env
             sug = suggest_commands(cmd0, env, builtins.aliases)