Merge branch 'v4.0.0_patches' into 'master'

update versions

See merge request zcash-docs/zcash-docs!205

source/rtd_pages/Debian-Ubuntu-build.md

@@ -27,7 +27,7 @@ There are multiple ways to download dependencies and build Zcashd & Zcash-cli. W
     ```
      git clone https://github.com/zcash/zcash.git
      cd zcash/
-     git checkout v3.1.0
+     git checkout v4.0.0
      ./zcutil/fetch-params.sh
     ```
 

source/rtd_pages/Linux-misc-build.md

@@ -41,7 +41,7 @@ Currently, the only way to install Zcashd & Zcash-cli on Linux is to build from
     ```
      git clone https://github.com/zcash/zcash.git
      cd zcash/
-     git checkout v3.1.0
+     git checkout v4.0.0
      ./zcutil/fetch-params.sh
      ```
      <!--The message printed by ``git checkout`` about a "detached head" is normal and does not indicate a problem. --> 

source/rtd_pages/OSx-build.md

@@ -34,7 +34,7 @@ Currently, the only way to install Zcashd & Zcash-cli on OSx is to build from so
     ```
      git clone https://github.com/zcash/zcash.git
      cd zcash/
-     git checkout v3.1.0
+     git checkout v4.0.0
      ./zcutil/fetch-params.sh
      ```
      <!--The message printed by ``git checkout`` about a "detached head" is normal and does not indicate a problem. --> 

source/rtd_pages/zcashd.md

@@ -67,9 +67,9 @@ To upgrade Zcashd & Zcash-cli, fetch the latest updates:
 Run Zcashd again and verify that the version number matches that of the [source repository](https://github.com/zcash/zcash).
 
 Helpful tips: 
-* Upgrading to v3.0.0 may require a re-fetch of the parameters and a build; try the following: 
+* Upgrading to v4.0.0 may require a re-fetch of the parameters and a build; try the following: 
   ```
-  git checkout v3.0.0
+  git checkout v4.0.0
    ./zcutil/fetch-params.sh
    ./zcutil/build.sh -j$(nproc)
   ```