This patch (primarily) documents the VEC_PERM_EXPR tree code in
generic.texi.  For ease of review, it is provided below as a pair
of diffs.  The first contains just the new text added to describe
VEC_PERM_EXPR, the second tidies up this part of the documentation
by sorting the tree codes into alphabetical order, and providing
consistent section naming/capitalization, so changing this section
from "Vectors" to "Vector Expressions" (matching the nearby
"Unary and Binary Expressions").

Tested with make pdf and make html on x86_64-pc-linux-gnu.
The reviewer(s) can decide whether to approve just the new content,
or the content+clean-up.  Ok for mainline?


2023-02-04  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* doc/generic.texi <Expression Trees>: Standardize capitalization
	of section titles from "Expression trees".
	<Language-dependent Trees>: Likewise standardize capitalization
	from "Language-dependent trees".
	<Constant expressions>: Capitalized from "Constant Expressions".
	<Vector Expressions>: Standardized section name from "Vectors".
	Document VEC_PERM_EXPR tree code.  Sort tree codes alphabetically.


Thanks in advance,
Roger
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