More updates to Building chapter of Emacs manual.

More updates to Building chapter of Emacs manual.

* doc/emacs/building.texi (Executing Lisp): Fix xref for C-M-x.
(Lisp Libraries): Add xref to node explaining `load' in Lisp
manual.  Note that load-path is not customizable.
(Lisp Eval): Note that listed commands are available globally.
Explain the meaning of "defun" in the C-M-x context.
(Lisp Interaction): Copyedits.
(External Lisp): Fix name of inferior Lisp buffer.  Mention Scheme.
(Compilation): Define "inferior process".