Survival endpoint descriptions update

[Rima-Waleed]

Cancer studies updated in this pull request:

• acc_tcga_pan_can_atlas_2018
• blca_tcga_pan_can_atlas_2018
• brca_tcga_pan_can_atlas_2018
• cesc_tcga_pan_can_atlas_2018
• chol_tcga_pan_can_atlas_2018
• coadread_tcga_pan_can_atlas_2018
• dlbc_tcga_pan_can_atlas_2018
• esca_tcga_pan_can_atlas_2018
• gbm_tcga_pan_can_atlas_2018
• hnsc_tcga_pan_can_atlas_2018
• kich_tcga_pan_can_atlas_2018
• kirc_tcga_pan_can_atlas_2018
• kirp_tcga_pan_can_atlas_2018
• laml_tcga_pan_can_atlas_2018
• lgg_tcga_pan_can_atlas_2018
• lihc_tcga_pan_can_atlas_2018
• luad_tcga_pan_can_atlas_2018
• lusc_tcga_pan_can_atlas_2018
• meso_tcga_pan_can_atlas_2018
• ov_tcga_pan_can_atlas_2018
• paad_tcga_pan_can_atlas_2018
• pcpg_tcga_pan_can_atlas_2018
• prad_tcga_pan_can_atlas_2018
• sarc_tcga_pan_can_atlas_2018
• skcm_tcga_pan_can_atlas_2018
• stad_tcga_pan_can_atlas_2018
• tgct_tcga_pan_can_atlas_2018
• thca_tcga_pan_can_atlas_2018
• thym_tcga_pan_can_atlas_2018
• ucec_tcga_pan_can_atlas_2018
• ucs_tcga_pan_can_atlas_2018
• uvm_tcga_pan_can_atlas_2018

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