org-element: Optimize working with match data and Emacs regexps

* lisp/org-element.el: Add commentary explaining some regexp-related
optimizations useful for the parser.

Link: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=63225
(org-element--headline-deferred):
(org-element-item-parser):
(org-element-item-interpreter):
(org-element--list-struct):
(org-element-plain-list-parser):
(org-element-example-block-parser):
(org-element-fixed-width-interpreter):
(org-element-paragraph-parser):
(org-element-src-block-parser):
(org-element-table-parser):
(org-element--parse-generic-emphasis):
(org-element-export-snippet-interpreter):
(org-element-link-parser):
(org-element--current-element):
(org-element--collect-affiliated-keywords):
(org-element-parse-buffer):
(org-element-normalize-string):
(org-element-normalize-contents):
(org-element--parse-to):
(org-element--cache-before-change):
(org-element--cache-for-removal):
(org-element-context): Avoid storing match data unless strictly
necessary.  Explain the necessity in places where we have to use
`save-match-data'.  Prefer `looking-at-p' (does not alter match data)
to `looking-at'.  Simplify regexps.  Update docstrings clearly
indicating when match data might be modified.
* lisp/org.el:
(org-offer-links-in-entry):
* lisp/ob-exp.el (org-babel-exp-process-buffer):
* lisp/org-agenda.el: Fix places where we need to protect match data.

lisp/ob-exp.el

@@ -173,7 +173,7 @@ this template."
 	    ;; buffer.
 	    (org-fold-core-ignore-modifications
 	      (while (re-search-forward regexp nil t)
-		(setq element (org-element-at-point))
+		(setq element (save-match-data (org-element-at-point)))
 		(unless (save-match-data
 			  (or (org-in-commented-heading-p nil element)
 			      (org-in-archived-heading-p nil element)))

lisp/org-agenda.el

@@ -6892,7 +6892,7 @@ scheduled items with an hour specification like [h]h:mm."
       (while (re-search-forward regexp nil t)
         (catch :skip
 	  (unless (save-match-data (org-at-planning-p)) (throw :skip nil))
-	  (org-agenda-skip)
+	  (save-match-data (org-agenda-skip))
 	  (let* ((s (match-string 1))
 	         (pos (1- (match-beginning 1)))
 	         (todo-state (save-match-data (org-get-todo-state)))

lisp/org.el

@@ -8537,7 +8537,7 @@ there is one, return it."
            ;; Only consider valid links or links openable via
            ;; `org-open-at-point'.
            (when (org-element-type-p
-                  (org-element-context)
+                  (save-match-data (org-element-context))
                   '(link comment comment-block node-property keyword))
 	     (push (match-string 0) links)))
 	 (setq links (org-uniquify (reverse links))))