Better document LATEX_HEADER_EXTRA keywords

* doc/org.texi (Header and sectioning): Add a footnote about the
  different between LATEX_HEADER_EXTRA and LATEX_HEADER.
* lisp/ox-latex.el (org-latex-classes): Be more explicit about
  LATEX_HEADER_EXTRA.

doc/org.texi

@@ -11389,9 +11389,12 @@ options have to be provided, as expected by @LaTeX{}, within square brackets.
 
 @cindex #+LATEX_HEADER
 @cindex #+LATEX_HEADER_EXTRA
-You can also use LATEX_HEADER and LATEX_HEADER_EXTRA keywords in order to add
-lines to the header.  See the docstring of @var{org-latex-classes} for more
-information.
+You can also use @code{LATEX_HEADER} and
+@code{LATEX_HEADER_EXTRA}@footnote{Unlike to @code{LATEX_HEADER}, contents
+from @code{LATEX_HEADER_EXTRA} keywords will not be loaded when previewing
+@LaTeX{} snippets (@pxref{Previewing @LaTeX{} fragments}).} keywords in order
+to add lines to the header.  See the docstring of @var{org-latex-classes} for
+more information.
 
 An example is shown below.
 

lisp/ox-latex.el

@@ -327,8 +327,8 @@ macro-like placeholders.
  [NO-DEFAULT-PACKAGES]   do not include any of the default packages
  [PACKAGES]              \\usepackage statements for packages
  [NO-PACKAGES]           do not include the packages
- [EXTRA]                 the stuff from #+LATEX_HEADER
- [NO-EXTRA]              do not include #+LATEX_HEADER stuff
+ [EXTRA]                 the stuff from #+LATEX_HEADER(_EXTRA)
+ [NO-EXTRA]              do not include #+LATEX_HEADER(_EXTRA) stuff
 
 So a header like