48888: improve doc for $$ and $PPID, add fail test for PPID readonly-…

…ness

ChangeLog

@@ -1,5 +1,8 @@
 2021-09-06  Bart Schaefer  <schaefer@zsh.org>
 
+	* 48888: Doc/Zsh/mod_system.yo, Doc/Zsh/params.yo, Test/E03posix.ztst:
+	improve doc for $$ and $PPID, add fail test for PPID readonly-ness
+
 	* 48832: Completion/Unix/Type/_urls: try _gnu_generic first
 
 2021-09-04  Oliver Kiddle  <opk@zsh.org>

Doc/Zsh/mod_system.yo

@@ -263,9 +263,9 @@ tt($$), which returns the process ID of the main shell process.
 )
 item(tt(ppid))(
 vindex(ppid, sysparams)
-Returns the process ID of the parent of the current process, even in
-subshells.  Compare tt($PPID), which returns the process ID of the parent
-of the main shell process.
+Returns the current process ID of the parent of the current process, even
+in subshells.  Compare tt($PPID), which returns the process ID of the
+initial parent of the main shell process.
 )
 item(tt(procsubstpid))(
 Returns the process ID of the last process started for process

Doc/Zsh/params.yo

@@ -654,10 +654,11 @@ Same as tt(#).
 )
 vindex($)
 item(tt($) <S>)(
-The process ID of this shell.  Note that this indicates the original
-shell started by invoking tt(zsh); all processes forked from the shells
-without executing a new program, such as subshells started by
-tt(LPAR())var(...)tt(RPAR()), substitute the same value.
+The process ID of this shell, set when the shell initializes.  Processes
+forked from the shell without executing a new program, such as command
+substitutions and commands grouped with tt(LPAR())var(...)tt(RPAR()),
+are subshells that duplicate the current shell, and thus substitute the
+same value for tt($$) as their parent shell.
 )
 vindex(-)
 item(tt(-) <S>)(
@@ -817,9 +818,9 @@ The operating system, as determined at compile time.
 )
 vindex(PPID)
 item(tt(PPID) <S>)(
-The process ID of the parent of the shell.  As for tt($$), the
-value indicates the parent of the original shell and does not
-change in subshells.
+The process ID of the parent of the shell, set when the shell initializes.
+As with tt($$), the value does not change in subshells created as a
+duplicate of the current shell.
 )
 vindex(PWD)
 item(tt(PWD))(

Test/E03posix.ztst

@@ -161,3 +161,6 @@ F:POSIX has neither math functions nor floating point
 0f:Width of %s is computed in bytes not characters
 F:This is considered a bugfix in zsh
 ><  Stéphane>
+
+  PPID=foo
+-f:PPID is not a readonly variable