Add documentation for \pgfkeysifassigned

doc/generic/pgf/CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## Unreleased
 
+### Added
+
+- Documentation of `\pgfkeysifassignable`
+
 ### Changed
 
 - Typo fixes in the manual

doc/generic/pgf/pgfmanual-en-pgfkeys.tex

@@ -280,6 +280,20 @@ \subsection{The Key Tree}
     %
 \end{command}
 
+\begin{command}{\pgfkeysifassignable\marg{full key}\marg{if}\marg{else}}
+    Checks whether this key was previously set using either |\pgfkeyssetvalue|,
+    |\pgfkeyslet|, |\pgfkeysdef|, or one of |\pgfkeysdef|'s relatives. If so,
+    the code in \meta{if} is executed, otherwise the code in \meta{else}.
+    %
+\begin{codeexample}[]
+\pgfkeysdef{/my family/my keyA}{\def\mykeyA{#1}}
+\pgfkeyssetvalue{/my family/my keyB}{Hello, world!}
+\pgfkeysifassignable{/my family/my keyA}{yes}{no}
+\pgfkeysifassignable{/my family/my keyB}{yes}{no}
+\end{codeexample}
+    %
+\end{command}
+
 
 \subsection{Setting Keys}