Add changes.txt and ltnews34 entries

base/changes.txt

@@ -6,6 +6,12 @@ completeness or accuracy and it contains some references to files that
 are not part of the distribution.
 ================================================================================
 
+2021-07-20  Phelype Oleinik  <phelype.oleinik@latex-project.org>
+
+	* lthooks.dtx:
+	Simplify \RemoveFromHook to not queue the removal of code chunks
+	that aren't yet in the hook (gh/625).
+
 2021-07-20  Phelype Oleinik  <phelype.oleinik@latex-project.org>
 
 	* ltcmdhooks.dtx:
@@ -16,11 +22,11 @@ are not part of the distribution.
 	* ltplain.dtx (section{Plain \TeX}):
 	Use 2 as value for \tracinglostchars so that one gets at least warnings
 	on the terminal (3 would be better as a missing char is an error)
-  
+
 2021-07-12  David Carlisle  <David.Carlisle@latex-project.org>
 
 	* ltclass.dtx:  fix for gh/617 prevent expansion in raw option list.
-  
+
 2021-07-11  Frank Mittelbach  <Frank.Mittelbach@latex-project.org>
 
 	* lterror.dtx (subsection{General commands}):

base/doc/ltnews34.tex

@@ -142,6 +142,30 @@ \section{???}
 
 \emph{write}
 
+\section{Hook business}
+
+\subsection{Changed how \cs{RemoveFromHook} treats code that isn't in the hook}
+
+In the first version of \cs{RemoveFromHook}, in case the code label
+being removed didn't exist in the hook, a ``removal order'' would be
+queued, and the next time something tried to add that label to the hook,
+the \cs{AddToHook} would be cancelled by the removal order, and no code
+would be added that once.  This was so that in principle package loading
+order wouldn't matter.  However this implementation didn't work quite as
+intended, because while two \cs{AddToHook} to a given label would be
+removed by a single \cs{RemoveFromHook}, one \cs{RemoveFromHook} could
+not cancel two \cs{AddToHook} to that label, and this asymmetry caused
+confusion and was a recipe for further problems.
+
+The implementation was changed and now \cs{RemoveFromHook} only removes
+labels that already exist in a hook, and will raise error if they don't.
+For usage across packages, for removing code in a hook, the
+\texttt{voids} relation should be used instead:  this relation is
+non-destructive (meaning it can be later reverted with another one), and
+it is truly independent of package loading order, so it should be
+preferred.
+%
+\githubissue{625}
 
 \section{New or improved commands}