stdenv: fix documentation for stripAllFlags and stripDebugFlags

The documentation for the default flags used for both fields were incomplete.

doc/stdenv/stdenv.chapter.md

@@ -886,7 +886,7 @@ Like `stripAllList`, but only applies to packages’ target platform. By default
 
 ##### `stripAllFlags` {#var-stdenv-stripAllFlags}
 
-Flags passed to the `strip` command applied to the files in the directories listed in `stripAllList`. Defaults to `-s` (i.e. `--strip-all`).
+Flags passed to the `strip` command applied to the files in the directories listed in `stripAllList`. Defaults to `-s -p` (i.e. `--strip-all --preserve-dates`).
 
 ##### `stripDebugList` {#var-stdenv-stripDebugList}
 
@@ -898,7 +898,7 @@ Like `stripDebugList`, but only applies to packages’ target platform. By defau
 
 ##### `stripDebugFlags` {#var-stdenv-stripDebugFlags}
 
-Flags passed to the `strip` command applied to the files in the directories listed in `stripDebugList`. Defaults to `-S` (i.e. `--strip-debug`).
+Flags passed to the `strip` command applied to the files in the directories listed in `stripDebugList`. Defaults to `-S -p` (i.e. `--strip-debug --preserve-dates`).
 
 ##### `stripExclude` {#var-stdenv-stripExclude}