Documentation for :export: tag usage

* doc/org.texi (Export settings): documentation describing how text
above the first heading is ignored when an :export: tag is in a file.

Commit 7cf4406088d4d7f50c35c6ba7ca3c351db2086bc introduced a feature of
the exporter that ignores text above the first headline when an :export:
tag is used anywhere in a file. This commit documents that behaviour
introduced in 7cf44060, in doc/org.texi.

TINYCHANGE

doc/org.texi

@@ -10519,7 +10519,8 @@ clocktable.
 The tags that select a tree for export (@code{org-export-select-tags}).  The
 default value is @code{:export:}.  Within a subtree tagged with
 @code{:export:}, you can still exclude entries with @code{:noexport:} (see
-below).
+below).  When headlines are selectively exported with @code{:export:}
+anywhere in a file, text before the first headline is ignored.
 
 @item EXCLUDE_TAGS
 The tags that exclude a tree from export (@code{org-export-exclude-tags}).