Updated code and docs to use gender neutral pronoun.

Used "singular they" based on Chromium's gender neutral code guideline.

Review: https://reviews.apache.org/r/58683

docs/committer-candidate-checklist.md

@@ -29,7 +29,7 @@ Demonstrated by: <input type="text"/>
 * Knowledge and activity breadth and depth in the Mesos project as well as areas where the candidate could be a maintainer:
 <input type="text"/>
 
-* Can the candidate be trusted to act the right way outside of known areas where he/she has already demonstrated committer-level expertise and behavior:
+* Can the candidate be trusted to act the right way outside of known areas where they have already demonstrated committer-level expertise and behavior:
 <input type="text"/>
 
 * High quality source code.

docs/versioning.md

@@ -40,7 +40,7 @@ Once an issue is deemed critical, it will be fixed in only those **affected** re
 
 Patch releases are normally done once **a month**.
 
-If a particular issue is affecting a user and the user cannot wait until the next scheduled patch release, he/she can request an off-schedule patch release for a specific supported version. This should be done by sending an email to the dev list.
+If a particular issue is affecting a user and the user cannot wait until the next scheduled patch release, they can request an off-schedule patch release for a specific supported version. This should be done by sending an email to the dev list.
 
 ## Upgrades
 

src/linux/cgroups.cpp

@@ -1200,7 +1200,7 @@ class Listener : public Process<Listener>
   // call to 'listen' returns a failure, all subsequent calls to
   // 'listen' will return failures as well (in that case, the user
   // should consider terminate this process and create a new one if
-  // he/she still wants to monitor the events).
+  // they still want to monitor the events).
   // TODO(chzhcn): If the user discards the returned future, currently
   // we do not do anything. Consider a better discard semantics here.
   Future<uint64_t> listen()

src/log/catchup.hpp

@@ -38,7 +38,7 @@ namespace log {
 // this function can provide a hint on the proposal number that will
 // be used for Paxos. This could potentially save us a few Paxos
 // rounds. However, if the user has no idea what proposal number to
-// use, he can just use none. We also allow the user to specify a
+// use, they can just use none. We also allow the user to specify a
 // timeout for the catch-up operation on each position and retry the
 // operation if timeout happens. This can help us tolerate network
 // blips.

src/tests/authorization_tests.cpp

@@ -2410,8 +2410,8 @@ TYPED_TEST(AuthorizationTest, LaunchNestedContainerSessions)
 
   {
     // "foo" principal cannot launch sessions as commands in any parent
-    // container. He may still get to launch container sessions if he
-    // is allowed launch nested container sessions whose executors are
+    // container. They may still get to launch container sessions if they
+    // are allowed to launch nested container sessions whose executors are
     // running as a given user for which "foo" has permissions and the
     // session uses a `container_info` instead of a `command_info`.
     mesos::ACL::LaunchNestedContainerSessionAsUser* acl =
@@ -3015,8 +3015,8 @@ TYPED_TEST(AuthorizationTest, LaunchNestedContainers)
 
   {
     // "foo" principal cannot launch nested containers as commands
-    // under any parent container. He may still get to launch nested
-    // container if he is allowed to do so for executors which run as
+    // under any parent container. They may still get to launch nested
+    // container if they are allowed to do so for executors which run as
     // a given user for which "foo" has permissions and the session
     // uses a `container_info` instead of a `command_info`.
     mesos::ACL::LaunchNestedContainerAsUser* acl =