Document new reserved keywords

Author: mcol

src/reference-manual/expressions.Rmd

@@ -110,7 +110,7 @@ the tokens are reserved for future use.
 
 ```
 for, in, while, repeat, until, if, then, else,
-true, false, target
+true, false
 ```
 
 Variables should not be named after types, either, and thus may not be
@@ -120,7 +120,7 @@ any of the following.
 int, real, vector, simplex, unit_vector, ordered,
 positive_ordered, row_vector, matrix,
 cholesky_factor_corr, cholesky_factor_cov,
-corr_matrix, cov_matrix.
+corr_matrix, cov_matrix
 ```
 
 The following block identifiers are reserved and cannot be used as variable names:
@@ -130,6 +130,11 @@ functions, model, data, parameters, quantities,
 transformed, generated
 ```
 
+Since Stan 2.22, the following identifiers cannot be used as variable names:
+```
+target, lower, upper, offset, multiplier
+```
+
 
 #### Reserved Names from Stan Implementation {-}
 
@@ -151,7 +156,7 @@ or `add`, but it may be named `pi` or `e`.
 Variable names will also conflict with the names of distributions
 suffixed with `_lpdf`, `_lpmf`, `_lcdf`, and `_lccdf`, `_cdf`, and
 `_ccdf`, such as `normal_lcdf_log`; this also holds for the deprecated
-forms `_log`, `_cdf_log`, and `_ccdf_log`,
+forms `_log`, `_cdf_log`, and `_ccdf_log`.
 
 Using any of these variable names causes the `stanc` translator
 to halt and report the name and location of the variable causing the