SA: Add comments about load address of executable

I removed the comment about load address of executable in JDK-8248876, but it contained useful information for maintenance.

So I re-add them, and add comment for the change in JDK-8248876.

```
diff --git a/src/jdk.hotspot.agent/linux/native/libsaproc/ps_core.c b/src/jdk.hotspot.agent/linux/native/libsaproc/ps_core.c
index dff4cb3eeca..9f7c4050ef2 100644
--- a/src/jdk.hotspot.agent/linux/native/libsaproc/ps_core.c
+++ b/src/jdk.hotspot.agent/linux/native/libsaproc/ps_core.c
@@ -507,7 +507,11 @@ static uintptr_t read_exec_segments(struct ps_prochandle* ph, ELF_EHDR* exec_ehd
         result = exec_php->p_vaddr;
         ph->core->dynamic_addr = exec_php->p_vaddr;
       } else { // ET_DYN
+ // Base address of executable is based on entry point (AT_ENTRY).
         result = ph->core->dynamic_addr - exec_ehdr->e_entry;
+
+ // dynamic_addr has entry point of executable.
+ // Thus we should subtract it.
         ph->core->dynamic_addr += exec_php->p_vaddr - exec_ehdr->e_entry;
       }
       print_debug("address of _DYNAMIC is 0x%lx\n", ph->core->dynamic_addr);
```