We no longer have two doc systems, move sphinx_doc_src back to doc_src

doc_src/cmds/exec.rst

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+.. _cmd-exec:
+
+exec - execute command in current process
+=========================================
+
+Synopsis
+--------
+
+::
+
+    exec COMMAND [OPTIONS...]
+
+Description
+-----------
+
+``exec`` replaces the currently running shell with a new command. On successful completion, ``exec`` never returns. ``exec`` cannot be used inside a pipeline.
+
+
+Example
+-------
+
+``exec emacs`` starts up the emacs text editor, and exits ``fish``. When emacs exits, the session will terminate.