Update PartitionAlloc comment

This CL updates the comment associated with enabling PA in PDFium,
making an effort to separate the explicit PA usage from PA-E (not
enabled at this time).

See also: https://crrev.com/c/3963582

Change-Id: I6fe8bf1950b92f85809e51c13bc3b442a75ccea7
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/99730
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

build_overrides/pdfium.gni

diff --git a/build_overrides/pdfium.gni b/build_overrides/pdfium.gni
index 8553398..6043a32 100644
--- a/build_overrides/pdfium.gni
+++ b/build_overrides/pdfium.gni
@@ -17,7 +17,8 @@
 # Default: Without XFA support.
 pdf_enable_xfa_override = false
 
-# Build PDFium with PartitionAlloc as the memory allocator.
+# Build PDFium with PartitionAlloc support, directing `fxcrt` to use
+# it as its memory allocator in lieu of `malloc()`.
 # Default: Use PartitionAlloc unless it is a MSVC build.
 pdf_use_partition_alloc_override = !is_win || is_clang