Update man files related to merge-base with better description

Change-Id: I9796d2034a8a3c1243defc7f18f0aee1d701ba00
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2414569Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>

man/html/depot_tools_tutorial.html

@@ -1115,9 +1115,9 @@ automatically run <code>gclient sync</code> to update your dependencies.
 </li>
 <li>
 <p>
-Use caution when running <code>git reset</code> as it may result in unexpected behavior of 
-<code>git rebase-update</code> and <code>depot_tools</code> in general - see 
-<a href="https://bugs.chromium.org/p/chromium/issues/detail?id=997395">crbug.com/997395</a>.
+Use caution when running <code>git reset</code> on branches managed by <code>depot_tools</code>.
+These tools track the set of "commits contained in the branch" from a branch&#8217;s "merge point"
+to the current branch value. See <code>git help mark-merge-base</code> for more discussion of the merge base.
 </p>
 </li>
 </ul></div>

man/man7/depot_tools_tutorial.7

@@ -420,7 +420,7 @@ Once you\(cqre done resolving all of the merge conflict, just run git rebase\-up
 .ps -1
 .br
 .sp
-Running git rebase\-update will update all your branches, but it will not automatically run gclient sync to update your dependencies\&. Use caution when running git reset as it may result in unexpected behavior of git rebase\-update and \fIdepot_tools\fR in general.
+Running git rebase\-update will update all your branches, but it will not automatically run gclient sync to update your dependencies\&. Use caution when running git reset on branches managed by \fIdepot_tools\fR. These tools track the set of "commits contained in the branch" from a branch\(cqs "merge point" to the current branch value. See git help mark-merge-base for more discussion of the merge base.
 .sp .5v
 .RE
 .SH "MANAGING MULTIPLE CLS"

man/src/depot_tools_tutorial.txt

@@ -280,7 +280,11 @@ you started on.
 
 [NOTE]
 Running `git rebase-update` will update all your branches, but it will not
-automatically run `gclient sync` to update your dependencies.
+automatically run `gclient sync` to update your dependencies. Use caution when
+running `git reset` on branches managed by 'depot_tools'. These tools track the
+set of "commits contained in the branch" from a branch's "merge point" to the
+current branch value. See `git help mark-merge-base` for more discussion of the
+merge base.
 
 
 MANAGING MULTIPLE CLS