Re: Fix wrong reference in pg_overexplain's doc

Hi  Fujii-san

> It would be more appropriate to use
> <filename>, <structname>, and <command> instead. 

```
-   following fields. See <literal>PlannedStmt</literal> in
-   <literal>nodes/plannodes.h</literal> for additional detail.
+   following fields. See <structname>PlannedStmt</structname> in
+   <filename>nodes/plannodes.h</filename> for additional detail.
```

Switching to <filename> here makes sense. While looking through the SGML files, 
I noticed that the way header file paths are written is a bit inconsistent across the documentation.

In many places we use full paths including src/include, for example in fdw_handler.sgml and create_type.sgml:

```
     in <filename>src/include/nodes/plannodes.h</filename>, and the comments for
     <type>ExecRowMark</type> in <filename>src/include/nodes/execnodes.h</filename> for
```

But in a few files, such as pgoverexplain.sgml (this patch), spi.sgml, 
and xfunc.sgml, the paths are written without the src/include/ prefix.

I’m fine with the change as-is; just wanted to ask whether you’d like to use 
this patch to address that inconsistency, or keep the existing style in this file.



Regards,
Shixin Wang.