INSTALL: reword and copy-edit the "libcurl" section

Make the "libcurl" section shorter and more to the point, this is
mostly based on suggestions from [1].

1. https://lore.kernel.org/git/YTtxcBdF2VQdWp5C@coredump.intra.peff.net/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

INSTALL

@@ -138,11 +138,11 @@ Issues of note:
 	  BLK_SHA1.  Also included is a version optimized for PowerPC
 	  (PPC_SHA1).
 
-	- "libcurl" library is used by git-http-fetch, git-fetch, and, if
+	- "libcurl" library is used for fetching and pushing
-	  the curl version >= 7.34.0, for git-imap-send. If you do not
+	  repositories over http:// or https://, as well as by
-	  use http:// or https:// repositories, and do not want to put
+	  git-imap-send if the curl version is >= 7.34.0. If you do
-	  patches into an IMAP mailbox, you do not have to have them
+	  not need that functionality, use NO_CURL to build without
-	  (use NO_CURL).
+	  it.
 
 	- "expat" library; git-http-push uses it for remote lock
 	  management over DAV.  Similar to "curl" above, this is optional