move para about write-env into write-env's itemdescr

[ericniebler]

a paragraph about write-env from P3396R1 got accidentally added outside of write-env's itemdescr. move it inside.

[jensmaurer]

The change seems fine, but it produces an overfull \hbox due to added indentation:

2025-01-12T04:18:15.0341238Z ##[error]Overfull \hbox (7.76248pt too wide) in paragraph at lines 1903--1908
2025-01-12T04:18:16.7916091Z ##[error]Process completed with exit code 1.

@ericniebler , please fix.

[ericniebler]

i tried to reproduce the problem locally and i cannot. "it works on my machine." then i tried splatting a bunch of hyphenation markup in some of the longer words. that didn't work. so i'm at a loss.

[jensmaurer]

Here's the overfull hbox:

I'll fix it.