Improve sig/checksum wording on rc template

Update the release candidate build script to improve the phrasing in the
release candidate email regarding cryptographic signatures and
references to checksums for Maven/Nexus tooling.

The intent is to make it more clear what purpose the sha1/md5 files
serve, and to more prominently highlight the cryptographic pieces that
are significantly more relevant to release prep and voting, but without
being excessively verbose.
diff --git a/assemble/build.sh b/assemble/build.sh
index a5251a3..d1810f9 100755
--- a/assemble/build.sh
+++ b/assemble/build.sh
@@ -132,18 +132,21 @@
 Staging repo: $(green "$stagingRepoPrefix-$stagingrepo")
 Source (official release artifact): $(green "$stagingRepoPrefix-$stagingrepo/org/apache/$tlpName/$projName/$ver/$projName-$ver-$srcQualifier.tar.gz")
 Binary: $(green "$stagingRepoPrefix-$stagingrepo/org/apache/$tlpName/$projName/$ver/$projName-$ver-bin.tar.gz")
-(Append ".sha1", ".md5", or ".asc" to download the signature/hash for a given artifact.)
 
-In addition to the tarballs, and their signatures, the following checksum
+Append ".asc" to download the cryptographic signature for a given artifact.
+(You can also append ".sha1" or ".md5" instead in order to verify the checksums
+generated by Maven to verify the integrity of the Nexus repository staging area.)
+
+Signing keys are available at https://www.apache.org/dist/$tlpName/KEYS
+(Expected fingerprint: $(green "$fingerprint"))
+
+In addition to the tarballs and their signatures, the following checksum
 files will be added to the dist/release SVN area after release:
 $(yellow "$projName-$ver-$srcQualifier.tar.gz.sha512") will contain:
 SHA512 ($(green "$projName-$ver-$srcQualifier.tar.gz")) = $(yellow "$srcSha")
 $(yellow "$projName-$ver-bin.tar.gz.sha512") will contain:
 SHA512 ($(green "$projName-$ver-bin.tar.gz")) = $(yellow "$binSha")
 
-Signing keys are available at https://www.apache.org/dist/$tlpName/KEYS
-(Expected fingerprint: $(green "$fingerprint"))
-
 Release notes (in progress) can be found at: $(green "https://$tlpName.apache.org/release/$projName-$ver/")
 
 Release testing instructions: $relTestingUrl